Battery Isolator design / operation

Submitted: Tuesday, Aug 30, 2011 at 15:59
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Hi all - been a while.

My question is "should I be concerned that my house batteries are continually being used for short high current discharges when starting the car or should I find a way to positively isolate the house batteries when the engine stops" - because on short stops the battery isolator is not dropping out - please read on on for explanation??

I expect the question I am asking here applies to all battery isolators (or voltage sensing relays by another name).

I have a major brand VSR which by specification cuts in at 13.3V and drops out at 12.8V and it does function properly within these limits - this one has no Override option so has only 3 connection points - Batt1, Batt2, and Earth - but that is proably beside the point of this discussion.

What I am finding is that on short stops the voltage of the batteries remains above 12.8V and so the VSR is not disconnecting - hence when starting the car again it is drawing from both the house and car batteries together (at least for a short time).

I would not have thought this was a good thing for the house batteries which are 2 x 120AH AGM Deep Cycle. If it is not a problem I guess I should not worry about it - but the cost of the batteries makes me want to know I am treating them the way they should be treated.

I can understand that the VSR is not going to drop out until the voltage drops to the pre-set level and this is by nature of the design and I am sure many of the Auto Engineers or similar people would be well aware of this situation.

For those with more pratical experience and knowledge of such VSR's, I was contemplating doing something that would momentarily disconnect the Earth from the VSR when the engine stops so the relay should drop out and not cut in again until the car starts and voltage comes up on the alternator side. This would effect immediate isolation of the house batteries. Not sure this is a practical solution and so looking for comments.


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Reply By: ABR - SIDEWINDER - Tuesday, Aug 30, 2011 at 16:06

Tuesday, Aug 30, 2011 at 16:06
Hi Ken

You will find that the isolator drops out the second you hit the key.

Let someone watch the LED on the isolator while you turn the engine over.

I would not be concerned or modify the unit in any way.


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Reply By: Member Al (Sunshine Coast) - Tuesday, Aug 30, 2011 at 16:29

Tuesday, Aug 30, 2011 at 16:29
Yes Ken, as Derek says, as soon as you engage the starter, the voltage on the cranking battery will drop below the 12.8v drop-out setting and the isolator will disconnect the house battery. There could be a few milliseconds of current drawn from the house battery but it would not have time to rise to any significant value. It is of no concern.


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Reply By: Meggs - Tuesday, Aug 30, 2011 at 16:42

Tuesday, Aug 30, 2011 at 16:42
Don't worry about it as I modified my isolator so I could switch it on and off from the dash and I also put an LED in for indication as as soon as the starter engages the isolator drops out.
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Reply By: Crackles - Tuesday, Aug 30, 2011 at 16:43

Tuesday, Aug 30, 2011 at 16:43
There seems to be a perception that if you start a car off a deep cycle battery that it will somehow damage it & lead to premature failure. My experience is the opposite in fact I don't even have a dedicated starting battery instead cranking off 2 deep cycles, so in that light I seriously doubt for the brief moment the car is started that there could ever be a problem particually as the voltage would most likely drop below 12.8 volts anyway causing the isolator to open off.
Cheers Craig.................
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Follow Up By: Member Al (Sunshine Coast) - Tuesday, Aug 30, 2011 at 19:22

Tuesday, Aug 30, 2011 at 19:22
Well Crackles, deep cycle batteries are really not designed to repeatedly handle the currents that starter motors demand so its not real good for them. Hence the perception. If you are cranking from two deep cycles in parallel and they are interconnected with heavy cable then at least the shared load reduces the demand on each battery.
It's a bit like snatching from a towball....... plenty of people do it and get away with it but then again...............???

But take my old 1HZ. It doesn't exactly leap into life when cold and cranks away for some few seconds drawing lots of amps so I'd rather not be feeding it from deep cycles. What are you cranking up Crackles? Mind you if I were in the middle of nowhere with a dead cranker I wouldn't hesitate to crank from my deep cycles, although I would probably open the breaker from my expensive AGM and leave it to the flooded deep cycle.


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Follow Up By: Crackles - Tuesday, Aug 30, 2011 at 20:54

Tuesday, Aug 30, 2011 at 20:54
Poor analogy Al as it's nothing like snatching from a towball.
I was told starting off deep cycles would kill my batteries in less than 6 months. My response was they were already 4 years old so asked for a revised estimate of when I could expect them to die. Coming on 6 years now for this pair so maybe you could hazard a guess? ;-) I believe the idea originated from Police S&R crews who replaced the starter batt with twin Deep cycles in their 80 series Cruisers to run additional radios/lighting etc.
I use 2 large deep cycles wired as you say to start my TD 105 series & 2 AGM's in parrallel to start my Wrangler (both with electric winches). In my circle of friends this is a very common arrangement & none have had a premature failure of a deep cycle battery ever! What they all have had is premature failure of various isolating systems & is one of the reasons we no longer use them. I suppose we could pinch your snatching with towball analogy here to compare with isolators as it's only a matter of time before the dust, heat & vibration leads to many of them failing.
In any case the power being shared by the House batteries in Kens example (between 40 & 60 amps) would be well within their limits.
Cheers Craig........
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Follow Up By: Crackles - Tuesday, Aug 30, 2011 at 22:03

Tuesday, Aug 30, 2011 at 22:03
Quote from Trojan site FAQ:
"Can I use my Deep Cycle battery as a starting battery?"

Deep cycle batteries can be used for engine starting but starting batteries should not be used for deep cycle applications. A deep cycle battery may have less cranking amps per pound than a starting battery, but in most cases a deep cycle battery is still more than adequate for the purpose of starting an engine.

In my case I have twin 130 ah Deep Cycle batts & a combined Cranking Amps of 1640 (1330 CCA). With a conservative total of 240ah I have 150 ah available for the fridge & lights when using the top 60% of charge. How does that compare with all you folk using Isolated twin battery systems & are you getting 6+ years life out of them? A simple system that simply works.
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Follow Up By: Member - Captain (WA) - Tuesday, Aug 30, 2011 at 22:38

Tuesday, Aug 30, 2011 at 22:38
Hi Craig,

The proof is in the pudding, you cannot argue with your success. You have chosen wisely the type of deep cycle battery and that they are CCA rated too.

I would still head Allan's good advice that typically deep cycle batteries are not made for starting as many deep cycle batteries would quickly die in that type of setup. I know this from bitter experience - many, many years ago I lost a deep cycle battery in less than 6 months, but it was a "true" deep cycle and not CCA rated at all. Sometimes you learn more from your mistakes...


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Follow Up By: Crackles - Tuesday, Aug 30, 2011 at 23:16

Tuesday, Aug 30, 2011 at 23:16
I too had a couple of early failures Captain using the old NZ70 sized Deep Cycles (85ah) & probably of dubious quality. Too much winching & cycling to near flat soon teaches you how much power you really need. I suppose buying the highest rated quality battery you can fit is always a good start.
On my next vehicle I'll be tempted to try a pair of Trojan 100ah deep cycle AGM's. Bit less capacity but a far quicker recharge time which may suit my touring style.
Cheers Craig............

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Follow Up By: Member Al (Sunshine Coast) - Tuesday, Aug 30, 2011 at 23:45

Tuesday, Aug 30, 2011 at 23:45
Well Craig, something that I have learned in engineering is that despite all the manufacturer's specifications, despite all the 'rules', there are always examples of someone successfully defying them, as you have extolled. Nevertheless, battery manufacturer's generally advise against using deep cycle for cranking, other than the compromise type.
In your case of two large deep cycles in parallel, the cranking burden is shared so that each battery is delivering only half the starter current, and therein I think lies your secret to success. Furthermore, I expect the TD105 to fire up fast enough to minimise the strain on the batteries. But my old 1HZ would really test the friendship.
I actually have a flooded cranker, and flooded deep cycle in parallel with an AGM in the troopy, which is working fine, but I have toyed with the idea of 3 AGM's in permanent parallel.

Above all though, it is really poor to use cranking type batteries for deep cycle application. They don't like it at all.

And I actually said it's a BIT like snatching from a towball, some get away with it, some don't!


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Follow Up By: Battery Value Pty Ltd - Tuesday, Aug 30, 2011 at 23:55

Tuesday, Aug 30, 2011 at 23:55
Hi there,

I think it's about time to enter this discussion and shed some light on what AGM batteries can actually be used for, and under what conditions.

Both deep cycle and normal duty AGM batteries when fully charged, can handle starter currents quite well, with only minimal voltage drop during cranking.
The reason why AGM battery manufacturers rate their product as either deep cycle or starter battery has something to do with the sealed thermoplastic battery container.

Some common usage patterns are:

User A has a deep cycle AGM battery mounted under the bonnet and its use is primarily for starting and very shallow cycling with minimal Ah demand before the battery gets recharged again by the alternator. The charging current stays low for most of the time except for a few minutes after cranking.
User A is a happy person as the battery still performs its task well for a number of years now (the capacity has been dropping away steadily but who cares - starting only requires 0.5Ah anyway).

User B has his AGM battery mounted in the trunk, and he takes advantage of the battery's deep cycle properties and discharges it to 30% SOC every single night before recharging it by combined alternator/solar during the day.
User B is also happy about the battery's performance since it's now 2 years old.

User C has the deep cycle AGM battery mounted under the bonnet, short and heavy wiring to the alternator, AND likes to discharge the battery to below 50% SOC on a nightly basis.
For the first few months everything's working fine, but suddenly his battery goes tits up and might have even deformed badly.
He's wondering as to why this has happened although the battery shows excellent deep cycle properties, plus respectable cranking amps in the specs sheet.

Long story cut short:
use your deep cycle AGM battery for cranking AND for deep cycling, only if the charging current stays below the specced maximum, AND the battery temperature below 45 degrees.
Both these conditions cannot be met for an under bonnet mounted battery.

And this is the number one reason why AGM battery manufacturers usually don't rate their batteries as deep cycle AND starter battery (unless their marketing department is of different opinion of course).

BTW flooded type batteries don't accept charge at a decent rate either, under these conditions, they just don't fail catastrophically early in their life because they are vented.

Hope this clears up some misperceptions.

cheers, Peter
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Follow Up By: Member Al (Sunshine Coast) - Wednesday, Aug 31, 2011 at 00:34

Wednesday, Aug 31, 2011 at 00:34
All very well as it goes Peter, however it could be a bit difficult to position an AGM
within reasonable reach of the starter motor without subjecting it to a temperature above your nominated 45 degrees. Perhaps if it were well heat shielded and ventilated it could be positioned in the engine bay?
Second question...... how do you ensure that your vehicle's charging system stays below the specced maximum?


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Follow Up By: Member - Captain (WA) - Wednesday, Aug 31, 2011 at 00:43

Wednesday, Aug 31, 2011 at 00:43
Umm... Peter, I am not sure what the "sealed thermoplastic container" has got to do with being a starter or deep cycle battery? The actual construction inside is what determines the usage type.

As a generalisation, starter batteries have many thin plates per cell, this gives a high surface area so a high instantaneous current draw. Deep cycle batteries generally have less plates, but much thicker. This gives less instantaneous current draw, but the thicker plates allow a longer draw of the lower current.

Flooded batteries are vented as the gases generated during the recharge cycle have to go somewhere. The gases generated are basically water (H2 and O2) and is why flooded batteries need to be periodically topped up with distilled water.

AGM batteries, a class of VRLA (valve regulated lead acid), are sealed but vent their gases if the pressure builds up greater than 1-2psi. Normally the gases (H2 and O2) recombine under the 1-2psi pressure and there is no "water" lost. However, if they are charged at too great a rate, or if the (underbonnet) temp is too high, then gases are produced faster than they can re-combine and the batteries vent. Any gas vented is permanetly lost and leads to a loss of battery capacity. Under overcharge conditions, AGM's do NOT swell, its only if their vents are blocked that this occurs.

Then you get the spiral AGM's (Optimas) where the vent pressure is in the 40psi range. To handle this pressure the plates are wrapped in spirals and are tightly wound - spiral AGM's have much better vibration resistance. The higher pressure allows a higher recharge rate as they have a higher gas recombination rate.

Anyway, this is but a very, very brief overview of some batteries, whole books are written on this subject. But to reiterate, its the internal construction and chemistry type of the batteries that determines what its optimal usage should be. To get the best battery life, the approriate type should be chosen for its application. And I agree that keeping charge current and temp below the batteries rated maximum is key to long battery life - amongst many other things!


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Follow Up By: Battery Value Pty Ltd - Wednesday, Aug 31, 2011 at 08:48

Wednesday, Aug 31, 2011 at 08:48

I agree it could be difficult to modify your rig to position the starter battery away from the heat.
That's why some car makers do this already in the design stage (OKA, Holden, BMW just to name a few) - and commercial trucks with their 24V system negating the need to mount their battery close to the starter motor.

And if the wires are short, the only way to keep below the max charging current is by active regulation.
BTW short bursts of moderately excessive charging current especially while the battery is still cool cant't do much harm.
But excessive charging current of longer duration contributes to AGM battery failure under high ambient, pushing the battery into an unstable region where there's a point of no return.

At this stage the oxygen recombination rate would have risen as well and because it's exothermic, there's this self feeding loop which ends catastrophically.
E.g. at a typical alternator voltage of 14V this unstable region could start at 52 degrees.


as you would know, thermoplastics soften with rising temperature.
And because AGMs have this internal working pressure of 4~6psi, and this additional gas recombination heat source, the battery container could deform before the overpressure relieve valves activate.
And because a starter battery has to endure high temps (cause the designers chose to mount it under the bonnet), then it's got to be of the vented type for this reason.

It's as easy as that really.

cheers, Peter

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Follow Up By: Member - Captain (WA) - Wednesday, Aug 31, 2011 at 09:54

Wednesday, Aug 31, 2011 at 09:54
Peter, if you genuinely believe that the type of plastic case of a battery is what determines a battery as starter or deep cycle, well I am at a loss for words...

Thermoplastics can be designed to have a very wide range of operational temperatures, just a matter of cost. Its the internal mechanical layout and chemistry of the battery that determines its designated type.

As an example, take the Optima battery, arguably one of the best batteries. The red top and yellow top appear to have similar case construction, yet the red top is warranted for starter use but NOT deep cycle use. The yellow top is warranted for starter AND deep cycle use. Surely its the inside mechanical differences that make the difference, not the plastic case?



PS. ALL lead acid batteries are vented, some directly and others via pressure valves - but ultimately still a vented battery.
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Follow Up By: Member Al (Sunshine Coast) - Wednesday, Aug 31, 2011 at 10:06

Wednesday, Aug 31, 2011 at 10:06
So Peter, if as you say, "it could be difficult to modify your rig to position the starter battery away from the heat." and "the only way to keep below the max charging current is by active regulation", is it appropriate to suggest to owners of existing vehicles that it can be a proposition to use AGM batteries for cranking? Pertinent design maybe for new vehicle manufacturers but hardly practical for existing vehicles. So the whole proposition becomes largely academic.

However, perhaps this whole subject is conceptual as Craig (Crackles) has declared an extended life of for AGM's cranking his Wrangler and has not expressed any special arrangements for temperature or charge current limitation. What is the strategy Craig, how have you beaten the pundits?


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Follow Up By: Battery Value Pty Ltd - Wednesday, Aug 31, 2011 at 11:39

Wednesday, Aug 31, 2011 at 11:39

I'd like to invite you to re-read my original contribution.
I think I may have confused you by saying:
..The reason why AGM battery manufacturers rate their product as either deep cycle or starter battery has something to do with the sealed thermoplastic battery container...right?

To clarify this for you one more time:
you can use any AGM battery (of sufficient size), for either cyclic apps, OR starting, but not for BOTH under high ambient/high charging current conditions.

The reason why not for BOTH, has something to do with the sealed thermoplastic container (and other things which may be even more confusing).
BECAUSE if used in a cyclic app AND starting CONCURRENTLY, there's an INCREASED risk to hit a condition called THERMAL RUNAWAY which causes the THERMOPLASTIC container to deform under high ambient/uncontrolled charging current situations, such as under the bonnet.

I didn't say that the type of plastic container determines a starter or deep cycle battery.

And don't think for one moment spiral wound AGM batteries are immune to this - google it if you want.
They just enter this condition a little later due to their different makeup.

Because Op.ima is very well aware of this, they make double sure that the internal resistance is being kept low in all cells (something which counteracts TR) by recommending a complicated charging routine.
Only then, the risk of hitting TR is minimised sufficiently to enable them to market their product as both deep cycle and starting capable concurrently.

Chemistry and its application in an AGM battery isn't black and white, it's a mixture of probabilities governed by affinities and temperature, mainly in two reactions and a great number of side reactions, still not all understood in all detail.

And that's all there's to it - sorry no more comments.


yes it's appropriate to suggest a deep cycle battery to be used for cranking if installed outside the bonnet.
Doing so gets rid of the temperature problem which is at the root of a number of battery failure types.
Excessive charging currents can then be tolerated for a longer time.

And last but not least, I should mention there's an easy way to get rid of excessive charging current as well:
install two or more deep cycle batteries in parallel :)

cheers, Peter
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Follow Up By: Member - Captain (WA) - Wednesday, Aug 31, 2011 at 13:04

Wednesday, Aug 31, 2011 at 13:04

You normally have a very good and accurate contribution to electrical threads and your position as a retailer of batteries lends credibility to your comments. However, in that same vein, you need to be aware of posting information that may misleading to those who do not have as good an understanding of the subject as yourself. I am not trying to denigrate your contribution, just trying to correct what I saw as a potentially misleading comment.

Your comment ”... The reason why AGM battery manufacturers rate their product as either deep cycle or starter battery has something to do with the sealed thermoplastic battery container...” is potentially misleading as you did not mention one thing about internal construction, just the plastic case. It implies, and perhaps you did not intend this, that the case is what determines a battery type.

The original post, which seems to have dropped by the wayside, was about the isolator not disconnecting the deep cycle battery during starting. A good question was asked, especially as it came to light that the Original Poster (OP) was blowing 40A fuses. As you would know, there is range of batteries from starter to deep cycle and every type in between.

As the old saying goes “oils ain’t oils” also applies to deep cycle batteries. Some types would be damaged by a regular 40A discharge current while others would not be fazed. Point being it’s hard to answer accurately the original question without knowing the exact “deep cycle” battery type.

Crackles (Craig) comments how he successfully uses deep cycle batteries for starting, but his batteries are also CCA rated so not an issue for HIS batteries – they are not specifically a deep cycle but really a hybrid type with their CCA rating and a/hr rating. Then Alan comments correctly IMHO that deep cycle batteries (not hybrids) are not designed for starting. And so the thread continues with what are mostly good comments, but should really contain some caveats as they can be taken out of context (like the plastic case...).

It’s a pity that any electrical thread on any forum tends to degenerate into a “mines bigger than yours” ego bashing about how much one knows, as I believe all posters are genuinely trying to help the OP. But what seems to be a seemingly innocent question often ends up on a tangent as assumptions are made when really more info is needed to answer the original question properly.

One issue with electrical threads is that there is more than one way to do things, while many ways will get you there, some are better than others and there are just as many opinions on what they are! Hopefully those that do have a good practical and theoretical electrical knowledge can try and keep being productive in their answers and not get sucked into ego bashing – me included!


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Follow Up By: Crackles - Wednesday, Aug 31, 2011 at 18:20

Wednesday, Aug 31, 2011 at 18:20
"(Crackles) has declared an extended life for AGM's cranking his Wrangler and has not expressed any special arrangements for temperature or charge current limitation. What is the strategy Craig, how have you beaten the pundits?"
I'm certainly not claiming any longevity just yet for the JK Wrangler Captain as it's only a touch over 3 years old. You may be aware they come standard from the factory now with Yellow top Optima's to start & with no more space under the bonnet was forced to fit my 105ah AGM under the rear of the car in a purpose built tub. This fixed the heat problem for the one in the back but as there was a steady number of Optima failures being reported I decided to install the AEV heat reduction bonnet (Vented) as I'd seen similar problems in the old 70 series Cruisers caused by the narrow grills, tight engine bay & the battery too close to the exhaust. (Toyota fixed their later models with vents & an insulated heat shield)
As far as limiting current there are 2 batteries joined together at all times so asume they share half the charge, that & the wife drives around with the stereo turned up loud ;-)
Cheers Craig............
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Follow Up By: Member - Captain (WA) - Wednesday, Aug 31, 2011 at 19:02

Wednesday, Aug 31, 2011 at 19:02
Umm... Craig, it was Allan who asked the question of you about your AGM extended life :)


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Reply By: KenInPerth - Tuesday, Aug 30, 2011 at 17:07

Tuesday, Aug 30, 2011 at 17:07
Thanks to all so far with quick responses

The reason I know it is taking something from the house batteries is that (perhaps being over cautious) I put 40A fuses on each battery take-off and these were blowing until I changed them up to 60A.

I will do a visual check on the VSR to see how quick it drops out and I guess this is what I would have expected - but as above there is certainly a long enough delay to blow the fuses - I am not sure what the reaction time of the fuses would be - dependent on how much above 40A it was as to how quick it would blow (maxi blade fuses).

I think I can deduce from the comments here is that the bottom line here is "don't worry about it".

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Follow Up By: Battery Value Pty Ltd - Tuesday, Aug 30, 2011 at 18:44

Tuesday, Aug 30, 2011 at 18:44
Hi Ken,

good observations.
Assuming 500A of starter current (which is at the higher end), and a 60/40 current split between starter and house batteries (actual ratio depends on factors like state of charge(s), wire resistances, age of batteries etc), the current spike through the isolation switch could be in the order of 200A.
Looking up the I2t curves of blade fuses, a 40A unit takes about 0.25 seconds to blow at this current.
This means the cutout time of the isolation switch would have to be longer than 0.25 seconds for a 40A fuse to blow.
You've done the right thing by selecting a higher rated fuse for the time being.

Just keep in mind that as the starter battery ages, the current spike through the isolation switch increases and there's a point in time when the 60A fuse will also blow.
So your idea of de-energising the isolation switch before starting has merit.

cheers, Peter
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Follow Up By: KenInPerth - Wednesday, Aug 31, 2011 at 00:37

Wednesday, Aug 31, 2011 at 00:37
Thanks Peter

This generated a lot more discussion than I expected but all good value.

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Reply By: Peter_n_Margaret - Tuesday, Aug 30, 2011 at 19:51

Tuesday, Aug 30, 2011 at 19:51
I am using a single Fullriver HGL 120-12 as a crank battery for the OKA.
It is not designed as a crank battery, but if you check the specs you will note that the maximum discharge current (5 seconds) is 1200 Amps. Lower currents would have higher acceptable times. This is plenty for cranking.

This battery has been totally reliable for 18 months to date. Previously I was destroying wet cells in 2 years due to vibration damage. That is unlikely to occur with an AGM. It also stays charged if unused for long periods.

OKA196 Motorhome.
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