Charging Dual Battery bank

fisher205

Explorer
After sitting all winter I decided the Alaskan needed some spring camping, but the house batteries were too low to ignite the furnace. They are 2 flooded cell Deep Cycle Everstarts I got last summer. I put the charger on one of the batteries and left it over night. Didn't work. Tried going through both batteries still didn't get enough to start the furnace. I also tried two different chargers. The smart charger kept cycling on and off. I was sure that I had ruined the batteries somehow. I drove the truck around on some errands since it had been parked all winter also. Tried the furnace and it worked. Seems the truck charges them fine.

So how am I screwing up charging a battery bank? I would like to get them fully charged before traveling my first trips are usually too short for the truck to charge the system.

Thanks, Brad
 

Tucson T4R

Expedition Leader
Hmm, this doesn't make much sense to me. I charge mine as well as keep them maintained with a smart charger attached across both batteries in the bank.

My experience with smart chargers is they will charge up a good battery much faster and more completely than the truck's charging system.
 

wrcsixeight

Adventurer
While a fully charged battery will not freeze till 40 below or something like that, the undercharged battery will freeze at warmer temps, and this might explain your current issues.

Even if the case did not crack, the cells might have shorted, or become highly resistive.

A hydrometer will shed some light on it.

Edit: just saw that the alternator is charging them. Will the other chargers take over after the alternator brings them into the high 12's?

A lot of chargers will not be able to charge a battery that is below 12 volts. The low battery must be paralleled with a good battery and then the charger should work.
 

NuggetHoarder

Adventurer
try hooking up a fully charged third battery to the dead batteries and let them equalize. Once some of the charge moves from the fully charged third battery into the dead batteries, the dead batteries will be able to take a charge from the charger.

Edit: Oops, just realized wrcsixeight posted the same advice first. My bad.
 

fisher205

Explorer
Thanks fo the info. I'll try charging them again. The blower would run on the furnace but it didn't have enough juice for ignition. So maybe I'll drive it some more and then try the charger.

Brad
 

NuggetHoarder

Adventurer
Driving isn't going to help if your batteries are drained super low. If they are drained that low, then the ONLY way to get some juice in them is going to be a fully charged THIRD battery - hooked up in parallel. Let the three batteries set for several hours so some juice from the third battery goes into the the two dead batteries, and then you can charge as usual. You may even want to split them up and do one battery at a time.

If your batteries won't accept any juice from the third battery, then they are probably toast.
 

pods8

Explorer
Sounds like you neglected your batteries all winter so far? You should throw them on a charger every so often when sitting idle...
 

McZippie

Walmart Adventure Camper
Some Smart Chargers don't work on dead batteries. To get a Smart Charger a to work on a dead battery, first put a charge in them using jumper cables from a fully charged battery. After a period of time, remove the jumper cables and see if the battery will take a charge using the Smart Charger.

If your Smart Charger has a "recondition mode" use it after the batteries are fully charged.
 

McZippie

Walmart Adventure Camper
Thanks fo the info. I'll try charging them again. The blower would run on the furnace but it didn't have enough juice for ignition. So maybe I'll drive it some more and then try the charger.

Brad
You'll probably be OK doing this, but I wouldn't, because may risk damage to vehicles charging system, if a neglected battery has a shorted cell, is sulfated or low on water etc.
 

Tennmogger

Explorer
This makes sense because an unused battery will often build some sulfation on the plates, increasing the internal resistance of the cells and battery. Charging them requires higher voltage to force current through higher resistance. A normal charger won't charge them (too low of voltage). Paralleling with another battery won't charge them (ditto).

The smart charger would apply voltage, sense no current (or small current indicating full charge) and turn off. Then battery voltage would drift down and the charger would turn on again.

In your vehicle, the charger/alternator does not care if there is load current or not. A constant voltage is applied to the dead batteries and they start taking a little charge. Many hours of this would probably charge your batteries...eventually. If the battery is drawing 2 amps, and for example the house battery is 100 AH, it'd take well over 50 hours to charge. That'd be a long drive.

What you need is a way to increase the voltage for initial charge. Might take 15-18 volts to force several amps of current into the battery. But then the sulfation starts to break down, resistance decreases, and the battery will start to take more current and voltage will drop. Once you achieve that, the smart charger can take over.

I use a heavy 24v transformer driven by a Variac, and with a single diode in the output. This simple charger will allow adjusting voltage as high as needed to make the battery draw current. This charger works on 24 v systems too.

A make-do setup could be made with two 12 v batteries in series to get 24v, then connect that across the dead battery through a 24v headlight in series. The headlight acts as a current limiter. Even if the dead battery is shorted there will be no issues, the bulb will just reach full brightness.

Bob WB4ETT


After sitting all winter I decided the Alaskan needed some spring camping, but the house batteries were too low to ignite the furnace. They are 2 flooded cell Deep Cycle Everstarts I got last summer. I put the charger on one of the batteries and left it over night. Didn't work. Tried going through both batteries still didn't get enough to start the furnace. I also tried two different chargers. The smart charger kept cycling on and off. I was sure that I had ruined the batteries somehow. I drove the truck around on some errands since it had been parked all winter also. Tried the furnace and it worked. Seems the truck charges them fine.

So how am I screwing up charging a battery bank? I would like to get them fully charged before traveling my first trips are usually too short for the truck to charge the system.

Thanks, Brad
 

Tennmogger

Explorer
Thanks for the replies! So can I put the charger on my truck batteries to charge the house batteries?
Do your truck batteries need charging too? If not, then don't over-charge them.

How are the house batteries connected to the truck system? Do they disconnect by relay when the truck is not running? Do you have a battery isolator/diode? etc.... A diode isolator will actually reduce charge voltage to the house batteries, compounding your (assumed) low charge voltage problem.

The problem is the non-maintained house batteries. You really need to concentrate on those separately, disconnected from the truck and camper. If higher voltage is needed to charge those house batteries then you need to avoid placing over-voltage on the camper system. And, disconnect the two batteries from each other, too, both for reconditioning, and during periods of disuse, even a few days. One or the other will be the weaker battery and will drain the better one.

Bob
 

fisher205

Explorer
Thanks again Bob, Will charge seperately. I guess I ask because as with all things these batteries are somewhat difficult to access and require removing the camper to pull out. Looking for the easy way out. I will try charging seperately and see how that works. I can remove the cables fairly easily. I'm not sure on how I would do a third battery yet.

Brad
 

Forum statistics

Threads
180,113
Messages
2,807,145
Members
215,360
Latest member
benzo
Top