Battery isolator upgrade?

Submitted: Sunday, Jun 17, 2012 at 21:22
ThreadID: 96332 Views:4268 Replies:2 FollowUps:9
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Just thought I'd ask if there's a better system, before replacing our automatic battery isolator which has failed. I recently noticed the car fridge runs both batteries down and the isolator is not protecting the starter battery.

We have a dual battery system in the car with an isolator and surge protection. Normally the car charges the second battery, and when the camper trailer is connected, charges its battery too via an Anderson plug.

It has worked fine for about 5 years, although a bit slow to charge as there is some voltage drop betwen the car and the other batteries when they are charging from low.

I have read a little about DC chargers and wonder if I should take the opportunity to "upgrade" or change the system, to achieve more efficient charging of the AGM batteries.
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Reply By: Sand Man (SA) - Monday, Jun 18, 2012 at 07:29

Monday, Jun 18, 2012 at 07:29
dc-dc chargers are certainly a worthwhile investment, especially for the remotely mounted batteries in the camper trailer. They have a built-in isolator so you will have adequate battery protection, at least for the auxiliary battery in the car.

For an isolator between the primary and auxiliary batteries, another dc-dc charger may be a bit over the top (cost wise), depending on where the auxiliary is mounted.
There are many quality dual battery isolators/controllers available and a good quality one is vital to protect your starting battery. Providing a heavy duty cable is used between the primary and auxiliary batteries, a smart isolator may be all that is required.

In my set-up, the primary battery is protected by the built-in controller on my Sidewinder Flyer AGM battery pack which sits in the back of the tub of the Colorado.
Another cable provides connection from the output of the Flyer, via an anderson connector, to another on the other end of the patch cable.
The camper cable is connected to this and the Ctek D250S Dual dc-dc charger mounted close to the remote batteries in the camper keeps these batteries in tip top condition, supported by a solar panel when stationary.


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Follow Up By: Gado - Monday, Jun 18, 2012 at 17:48

Monday, Jun 18, 2012 at 17:48
Thank you Bill, I'm still trying to understand some of the theory and other people's experience before deciding which way to go. A bit of searching and it gets quite complicated, and there are differing views out there.

Our "normal" travelling is plenty of driving most days and a solar panel for the occasional 2-3 day stop. The car's second battery and the trailer's battery have been adequate so far for the fridge and basic lighting.

I'm not yet convinced that the little extra charge that a dc-dc charger can add over the car alternator is going to be worth the hundreds of dollars it looks like costing.

Cheers, Gado
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Follow Up By: Sand Man (SA) - Monday, Jun 18, 2012 at 18:34

Monday, Jun 18, 2012 at 18:34

A dc-dc charger does not simply give "a little extra charge".
It ensures a speedy and complete charging process that is unlikely to be matched by a cable connection alone, even if the cables are heavy enough. Many of later model vehicles are restricted in the amount of charging current output from the alternator.

A dc-dc charger gives a higher charging current (depending on the brand and model of the charger) and provides a series of charging processes from bulk charging to pulse charging to ensure the expensive AGM batteries are charged fully in the quickest possible time. The charger will accept a lower input voltage and boost it to supply the correct level of voltage to the batteries.

In a long cable run, (eg to the camper batteries), the chances are that there will be sufficient voltage drop in the long cable run to limit full and efficient charging of the remote batteries. This can severely shorten the lifespan of the batteries as well as never charge them fully.

Have a look at the Ctek D250S or D250S Dual and perhaps the purchase price of a dc-dc charger can be better justified.

PS. I have no affiliation with Ctek. I'm just a satisfied customer.


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Follow Up By: Gado - Monday, Jun 18, 2012 at 19:30

Monday, Jun 18, 2012 at 19:30
Thanks again Bill for your patience, it's a steep learning curve for me...

From what I can read, the alternator itself should be OK and probably quicker for "bulk charging" i.e. the main job at start up, when both aux batteries are fairly low. For this, the Ctek output of 20A doesn't seem to be very much to charge 2 extra batteries from a 100+A alternator even if the voltage is a bit higher.

Once the batteries are fairly full again (after say an hour or two of driving) the Ctek output would no doubt come into its own for "topping up".

Another question - can the Ctek D250S Dual cope with both my auxiliary batteries at the same time (car and camper)? Or do I need two of them (they look like being over $300 fitting etc.)

Thanks, Gado
FollowupID: 763978

Follow Up By: Sand Man (SA) - Monday, Jun 18, 2012 at 21:15

Monday, Jun 18, 2012 at 21:15

You need to take both the charging current plus the charging voltage into account when assessing a solution.
20 Amps at an optimum level of 14.4 volts is adequate for an AGM battery bank of up to 300Ah.
If the alternator is only supplying a voltage of less than 14.4 volts the batteries will not charge fully or effectively, regardless of the current level.

As far as the D250S Dual coping with both car and camper batteries at the same time, no it won't. The best location for the DC charger is at the remote end of a long cable run, in your case the batteries in the camper and therefore the auxiliary battery in the car will only receive the voltage and current supplied from the alternator.
That is unless a smartpass system is incorporated into the equation. I do not have this option in my configuration but more info on the smartpass option can be accessed on the Ctek web site.

The cable from the starting battery to the auxiliary battery in the tub area of my dual cab is of sufficient capacity and shortish length, that I get a good level of voltage and current to charge the auxiliary. The D250s then receives whatever voltage output from the auxiliary battery (The 100Ah AGM battery in the Flyer) and boosts it if necessary, so that a voltage of 14.4v and a bulk charging current of 20 amps is available for the two 80Ah AGM batteries in my camper.

I haven't determined the need for an additional dc charger, or the smartpass device for the vehicle's auxiliary battery, but this battery does have an isolator to separate it from the charging battery.


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Follow Up By: Gado - Tuesday, Jun 19, 2012 at 08:52

Tuesday, Jun 19, 2012 at 08:52
Do I understand these principles right -

1. I presume a 20A output from the charger (like the Ctek D250S) would take nearly 30A from the input side (from alternator via VSR), and take about 3 hours to put 60ah into the battery.

2. Although more amps may be available direct from the alternator without the charger, the charging rate would be slower because of voltage drop, and the alternator voltage pressure isn't as high anyway.

The Ctek D250S plus smartpass kit would cost over $600 plus installation - it sounds like an expensive quest for some extra battery life and charging efficiency!

I'm thinking I should just replace the broken isolator (VSR) and then as a possible improvement, to look at a single DC charger next to the camper battery (120ah).

Cheers, Gado
FollowupID: 764017

Reply By: ABR - SIDEWINDER - Monday, Jun 18, 2012 at 19:40

Monday, Jun 18, 2012 at 19:40
Hi Gado

What do you drive ?

If it is a late model Toyota simply replace the isolator and fit a ABD to boost-the charge.


Derek from ABR
AnswerID: 488842

Follow Up By: Gado - Tuesday, Jun 19, 2012 at 08:16

Tuesday, Jun 19, 2012 at 08:16
Hello Derek, we can't afford Toyota, we have a 2006 Hyundai Terracan. It does what we want admirably and suits our budget...we are looking for a practical and cost effective result, even if it's not the latest and best.

I'm not sure if an alternator booster would achieve much. The alternator usually puts out about 14.4v for a while after startup and then settles back to about 13.8 while all batteries are charging. On the end of the line at the camper when its battery is low, it starts at about 13.2v and the difference reduces to very little (about 0.2 V) as the camper battery charge builds up.

Cheers, Gado
FollowupID: 764013

Follow Up By: ABR - SIDEWINDER - Tuesday, Jun 19, 2012 at 08:27

Tuesday, Jun 19, 2012 at 08:27
Hi Gado

Your voltages are good, make sure to use thick cable and replace the isolator, I see no need for a DC-DC charger if you have 13.8V on a hot engine. Good cable is cheaper than a DC-DC charger, our DC-DC charger is $249.00 and that does buy a lot of cable.


Derek from ABR
FollowupID: 764014

Follow Up By: Gado - Tuesday, Jun 19, 2012 at 10:17

Tuesday, Jun 19, 2012 at 10:17
Thank you Derek that sounds good to me. The autoelectrician who installed it insists the cable is up to scratch, I've had that discussion with him before, to reduce voltage drop and improve charging rate.

Cheers, Gado
FollowupID: 764025

Follow Up By: ben_gv3 - Tuesday, Jun 19, 2012 at 13:02

Tuesday, Jun 19, 2012 at 13:02
Ask your autoelec what size cable he actually used. You might be surprised what he considers to be adequate. From my readings on various forums, you'll need 6AWG (about 13mm2) or bigger if going from front to rear to reduce voltage drop which appears to be one of your problems (13.2V at your aux batt).
FollowupID: 764038

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