Low Voltage Diconnect (LVD)

Submitted: Wednesday, Mar 31, 2004 at 11:06
ThreadID: 11694 Views:3229 Replies:7 FollowUps:7
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I believe the fastest way to destroy a deep-cyle battery is to continually drop its depth of charge to below 50% to 40%.
Has anybody fitted a low voltage disconnect (LVD) system to the their caravan house battery so that it automatically cuts the power when the battery is discharged to 40%? Normally, you only know when you have discharged the battery when the lights go dim and this if done routinely, I am told, the battery life can be shortened to months.
I am interested to know if such devices are readily available and at what cost.
My budget won't stretch to solar panels at the moment, so I have depend on recharging the battery from the car when I travel. I have run heavy gauge cable (approx. 10 mm square) from the car auxilliary battery to the caravan via a 50 A Anderson plug.
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Reply By: knight44 - Wednesday, Mar 31, 2004 at 11:22

Wednesday, Mar 31, 2004 at 11:22

I recently fitted a low-voltage cutout in my 4WD easily enough. The one I bought was a Projecta brand unit (called a Sure Start), and if you decide to get one of these, shop around. Repco were selling them for over $40 (I've forgotten the exact price) but my local K-Mart had them for sale at $18.


AnswerID: 52629

Follow Up By: Rosscoe - Wednesday, Mar 31, 2004 at 11:41

Wednesday, Mar 31, 2004 at 11:41
Ta Richard,

I'll have a look around. Did you have to fit a solenoid as well or is the device fully opertional and able to handle the battery current.
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Follow Up By: knight44 - Wednesday, Mar 31, 2004 at 12:11

Wednesday, Mar 31, 2004 at 12:11

The unit is fully self-contained so no additional solenoid is required. It did come with cig plug & socket which I just cut off and then I hard-wired the unit into my circuit.

The max current it can handle is 10amps.

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Reply By: Rod - Wednesday, Mar 31, 2004 at 11:47

Wednesday, Mar 31, 2004 at 11:47
From memory, the Projecta unit switches up to 10A
AnswerID: 52633

Follow Up By: Member Colin - NSW Bungendore - Wednesday, Mar 31, 2004 at 22:37

Wednesday, Mar 31, 2004 at 22:37
I have a Deep cycle battery as the main - and only - battery in my Forester. I bought a deep cycle as my 3 way frig draws about 7A and I was told that this amount of drain would ruin a 'normal' battery? I use a Projector cut out unit on the frig and it cuts out at 11.6 V which is plenty enough power to start a 2.0 L petrol motor. I haven't timed it, but the battery will run the frig for about 2 hours before the Projector cuts in (out).
I also use 'hella' type plugs and I cut the wires back fairly short.
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Reply By: Crackles - Wednesday, Mar 31, 2004 at 17:48

Wednesday, Mar 31, 2004 at 17:48
Rosscoe. Most LVD units are adjustable and should be set around 11.7 to 11.9 volts to ensure a 30 % charge remains (depending on batt type etc). Obviously it cuts your usable power by 30% + but will ensure a longer battery life. They are not for switching heavey loads and fitting a relay would only deplete the batt more.
Cheers Craig................
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Reply By: David N. - Wednesday, Mar 31, 2004 at 21:39

Wednesday, Mar 31, 2004 at 21:39
you are correct in your assumptions about running ANY lead acid battery too flat too often.
To get the best charge setup without spending major dollars on aux charge systems, you need two (yes 2) separate cables from your alternator - one for running all items such as frig etc whilst driving, and one which has the sole function of charging the aux battery -whilst it is disconnected from the caravan auxillary items (again-frig etc.)
Think of it like this:
you have a regulated 13.8 to 14.2 volts at the alternator.
you take one lead to run frig/ / / etc and will probably get up to 2 volts drop depending on the load.... OK... frig will still work on 11.8 to 12.2v...
Now you run a separate lead from the alternator to CHARGE the BATTERY.... as the current drops, you will get damn close to your 13.8 or 14 volts at the aux battery and therefore as close to a fully charged battery as possible without going to expensive charge systems. (Then finally top-up your deep cycle battery the last bit if and whenever you can using a 240v charger or solar panel or whatever!)
I have beeen using this system very successfully for many years now on a number of vehicles and have had exceptional results, without blowing the budget!
PS In case you are wondering, I do know what I'm talking about as I have an electrical engineering degree, though now retired from that profession!!
Hope this helps, cheers and regards.
AnswerID: 52727

Reply By: ianmc - Wednesday, Mar 31, 2004 at 23:31

Wednesday, Mar 31, 2004 at 23:31
Not clear about that David! You run the frig etc OFF the alternator. What happens when you stop for a day or so!
Frig manufacturers are strong about running the frig direct from a battery only!
Maybe you can elaborate.

Also, I have just run a heavy cable from starter battery thru a 50amp fuse to the tray of my ute via a 50amp on-off switch, then another fuse at the second battery as adviused by auto elec.

Do you have any problems with this? Only one I can see apart from having to use the brain a bit, is what happens if the second battery is switched on when I crank over my diesel. Will it draw lots from battery 2 & blow the fuses & maybe overload the cables.??

Any comments appreciated......Ian
AnswerID: 52739

Follow Up By: daveread - Thursday, Apr 01, 2004 at 08:37

Thursday, Apr 01, 2004 at 08:37
What David is implying is that by running a separate feed cable for the fridge, it is only carrying the fridge current, reducing the volt drop in the cable allowing a higher voltage to be available for the fridge that would otherwise be the case.
The two cables are joined at the alt's output terminal, so the fridge still sees the battery.
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Follow Up By: Rosscoe - Thursday, Apr 01, 2004 at 10:00

Thursday, Apr 01, 2004 at 10:00
Thanks Guys,

The Projecta unit, as far as I can tell, is not adjustable but that's OK. I can use it as a warning signal and manually bypass it if necessary. I do run a 150W inverter from time to time (Notebook PC and TV) so I'll need to watch the 10A limit.
As David has inferred, there are two problems with lead acid batteries. Firstly, repeated deep discharge shortens their life dramatically and secondly, the normal alternator/voltage regulator system in the vehicle can only achieve about 80% charge. You can't fiddle with the regulator out put because modern computer based engine management sytems require a specific voltage level.

The LVD question I posed here and the answer is only part of the solution but non-the less, a very good way of getting reasonable economical life from my deep cycle battery.

Because we are dealing with a low voltage system voltage drop across cable is critical for effective operation of some appliances e.g. 3-way asorption fridges. I think I have this under control.
Now I'm going to start working on charging the batteries as close as practically possible to 100%.

Even with 240V AC available unless I use a smart charger I won't achieve this.

So it looks like shelling out more dollars on a microprocessor based smart charger (Approx. $300) and/or Solar panels with an appropriato solar controller (> $1000).
In the mean time I just have to be very carefull with the way I use power and limit the time I can stay in the bush without mains.

FollowupID: 314586

Reply By: ianmc - Thursday, Apr 01, 2004 at 11:51

Thursday, Apr 01, 2004 at 11:51
There are so many ways to skin a cat. Bet many auto elec's havent seen all the ideas on here about this topic.
Seems to me that my heavy cable (about 8mmsq) from starter battery to second battery in tray with fuses & in line on/off switch does something similar to Davids tapping into the alternator terminal which goes to the starter battery anyway.
The projecta charge transfer unit (which I have) is just another cheap & useful accessory to have if other stuff fails.
AnswerID: 52789

Reply By: Glenno - Thursday, Apr 01, 2004 at 18:24

Thursday, Apr 01, 2004 at 18:24
My Solstrum Solar regulator has a 3rd connection point which has a cut off at 40%. Might be a good investment if you are ever thinking of going solar down the track.


AnswerID: 52855

Follow Up By: Lynn2 - Thursday, Apr 01, 2004 at 20:40

Thursday, Apr 01, 2004 at 20:40

Can you please expand a little please.
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Follow Up By: Glenno - Friday, Apr 02, 2004 at 00:26

Friday, Apr 02, 2004 at 00:26
With my solar regulator it has three connections, one for Solar Cell, one for the batery and one for the output.

So the regulator not only handles the solar cell charging the battery, it also cuts off the output when the battery gets to 40%.

So if you dont have a Solar cell now but might get one in the future, you could use the regulator now and plug a solar cell on later.
FollowupID: 314687

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