View Full Version : How to recharge dual battery?

12-10-2009, 07:14 PM
I have a 2005 Isuzu NQR box truck. The two batteries are connected together. The batteries are low because I have to jump-start to get it going. I bought a PEAK 2-amp battery charger that auto-senses when the battery is fully charged and switches to trickle.
The problem is that the charger thinks that the batteries are fully charged because there are 2 batteries... so it doesn't charge... just trickles.

How can I charge these two batteries without removing them from the truck?

Can I use a dual-bank charger (like a Deltran Battery Tender) and connect each battery to a seperate bank of the charger or will this not work because the two batteries are wired together?

I read somewhere that there could be a "bridge switch" on the cable connecting the two batteries that I could flip to set the 2 batteries into parallel wiring... couldn't find such a switch.

Can anyone help? I'm sure someone has had a need to charge a dual-battery setup on a truck/boat/RV?

Thanks in advance!

12-10-2009, 07:25 PM
May seem like a dumb question but why cant you charge them one at a time? Disconnect the line that connects the two so there isnt a current between them that would trigger your charge to read theres enough battery power, and just charge them one at a time. When theyre all charge up properly just re connect the lines and you should be good to go, no?

12-10-2009, 07:59 PM
The reason that I'd rather not disconnect is because I only use the truck on the weekends. During the weekdays, I keep the truck in our fenced-off lot out back at night and move it to a streetside location during the day for advertising/visibility reasons.

Each time I start the truck, the starter battery is hit with about a 10% loss of charge to get the engine cranking. Driving the truck 300 feet from the back to the front is not enough to recharge the battery and recover the 10% loss. Over the span of 5 days of moving back to front and front to back, the battery is dead when I really need it at its fullest on the weekends.

What I'd like to do is plug it in on Thursday while it is sitting street-side so that it's nice and juiced for our big weekends. It'd be impractical to have to remove all of the protective covers to get to the wiring and unbolt it each week... possible, but I'd rather not risk a shock/short each week...

It'd be nice just to prewire the leads and do a quick-connect to the charger each Thursday.

I was thinking that I could wire a couple switches between each battery and flip the two switches to break the connection between the batteries when I needed to do a recharge... such a pain, but it's a possibility.

Anyone have luck with those solar chargers?

12-10-2009, 08:38 PM
With a little research, I've found the solution.

(1) Install battery isolator switch (Battery cut-off switch)

(2) Use Battery Tender 2-bank charger

12-10-2009, 09:11 PM
I don't know why the charger wont work the same way the truck draws juice from the battery's ?
2 amp is a trickle charger.
You should be able to hook one charger lead to the pos + terminal and the other Neg - lead to the frame of the truck .
It sounds like you need new battery's .
You should be able to start the truck more then 15 or 20 times .
I would try equalising the battery's.
On a warm morning leave your lites on and kill the battery's dead .
Then charge them with a 10 amp charger all night .
Kill the battery's dead and charge them 3 times If the battery's die again get new battery's . John

12-10-2009, 09:18 PM
You said you bought a 2-amp charger and say it is only trickle charging the batteries well 2 amp on those big batteries is like a trickle charge and will take a minimum of 12 hours to charge them.

The way i see it go get yourself a nice charger/starter with at least 200 amps of starting power. Throw that on there and it will start right up.

Or if you don't want to get that put a battery maintainer on them and it will keep them at peak power at all times.

Gravel Rat
12-10-2009, 09:27 PM
You don't want to use a trickle charger they are not good for the batteries. Get a good battery charger that has a crank setting and puts out a good charge. Once the batteries are charged use a solar charger to keep the batteries up.

12-10-2009, 10:06 PM
how about a remote start system,
some of these can be programmed to
start and run the vehicle for 20-30 minutes,
primarily to heat or cool or keep the engine warm,
but in your case would keep the batteries charges,
considering your electrical system is all functioning properly to
begin with

12-10-2009, 10:32 PM
You don't want to jump start the truck. It is not very nice on the glow plugs. Take the positive of one battery and connect it to the charger and on the opposite battery connect the negative of the charger. You need a amp gauge and see whats drawing the batteries down or drive the truck to the store and disconnect the batteries and have them test them.

12-10-2009, 10:55 PM
Any reason you cant just let it run for 5 minutes when you move it?

gene gls
12-10-2009, 11:33 PM
I had the same problem, let my Mitsi set for 4-5 days and something would drain the the batteries dead. I had a master switch put in, no more problems. Also acts as an anti thieft devise, just make sure you flip the switch every time you walk away from the truck.

12-11-2009, 12:45 PM
Thanks to everyone that replied to this thread! There were some very interesting perspectives and suggestions.

The reason I can't just plug the charger to the batteries is because when I do, the charger thinks that the batteries are at full power and refuses to stay in "charging mode" it auto-switches to "maintenance" mode. Not sure why, but I suspect it's because there are 2 batteries instead of the typical 1-battery setup. Honestly, I don't know for sure.

I've decided that the cut-off switch will be the easiest (and cheapest <$20) route. The charger will treat each battery independantly and will charge appropriately. I only need one, and will install it on the negative lead of one of the batteries. (see picture) http://www.amazon.com/Battery-Disconnect-Switch-Will-Both-Side/dp/B00099YOV8

I agree that the batteries may be bad, but I just replaced them about a year ago. I'll try discharging and recharging 3 times as one member has suggested.

Thanks again!


12-11-2009, 03:14 PM
sounds like a dead cell on a battery...it will show correct voltage with no load, but drops like a rock when a mild load is placed on it....

you need to use a voltmeter to check out your system - starting with "at rest" and work it back to the alternator....

Trickle chargers will do the job just by placing it on one of the battery terminals, and it will charge both batteries - however it takes up to 12 hours depending on battery health....battery tender is one brand that does it fine, many others....

12-11-2009, 03:52 PM

I'm assuming that a battery has multiple cells? Once I get it set up, will the charger be able to charge the battery with a "bad cell" or will I need to replace it?

Also, do you know how to tell which battery is the "starter battery" that is used to crank the engine, and which is the "accessory" battery used to power the liftgate?

Thanks for your help!

12-11-2009, 10:49 PM
If you have the batteries tested they can tell witch battery's dead .
The battery both work together for starting and accessories.

12-11-2009, 11:34 PM
The batteries are tied parallel so they work together.