combining battery groups a no no?

[AnnArborBob]

Originally Posted by lewster

My take,

With all of the battery experience that I have and considering what Justin @ Lifeline said in the boat article, I'm not convinced that different sized batteries (assuming all are Lifeline and of the same manufacture date) in a single bank will not draw at differing rates.

I sent Justin a message requesting this info based on his statements in the referenced article. We'll see what he comes back with (if at all).

Until I have empirical evidence of even draw and depth of discharge across all differing batteries in a mixed grouping, I'll continue to use identical batteries in my battery banks.

Remember, you still can't group different chemistries (AGM with gel or flooded cells) as their charging parameters vary widely.

The jury is still out!!!!!!


Lew Farber
RVIA/RVDA Nationally Certified Master Tech
ABYC Certified Master Marine Electrician
Master Tech Energy Systems, Inc.
AM Solar Certified Installation Center*AMS Lithium Batteries
Lifeline AGM Batteries**Magnum Inverters
541-490-6357

Thanks, Lew. Not that I have a dog in this fight, as you know, but I'm curious about the mixing of battery sizes for purposes of general knowledge.

[guskmg]

For what its worth, In my GM Duramax diesel are 2 OEM flooded cranking batteries. When my first battery failed while under warranty, GM replaced both batteries for the above compatibility reasons. Then about two years later one of those failed. They said it was now considered OK to replace the bad battery with a new battery without replacing the other if it tested OK. The warranty had expired so the cost to GM was not a factor. It has been three years and both batteries are still good. In the past year I was starting on 8 degree mornings with no difficulty. I was appalled at that advice and fix, but GM must have research the problem to make that determination. It will be interesting to see how long the odd combination lasts.
guskmg

[switz]

Even C & D cell flashlight batteries do not recommend different types be mixed, meaning chemical composition or extended live vs regular life.

[Rich W]

The charger in my boat is 3 separate banks, 15 amp each. The starting battery is group 27, trolling motor batteries are group 31. All are Deka flooded cell.

After a fishing trip I plug up the charger and each bank monitors the charge on each battery. The ignition battery will reach full charge in about 15 minutes, the trolling motor batteries will take several hours, same one always reaches full charge first.

I like the independent charging because one of the deep cycles always reaches full charge first, even when they are new (although the difference when new is maybe 5 minutes, over the years it grows to about 15 minutes.) If it charged the deep cycle batteries as a single charger it would cut off before one of them reached full charge. This seems like a good system to me, many of the on-board chargers are built this way now. I wonder why they don't do this for RV chargers.

[lewster]

Originally Posted by richw46

The charger in my boat is 3 separate banks, 15 amp each. The starting battery is group 27, trolling motor batteries are group 31. All are Deka flooded cell.

After a fishing trip I plug up the charger and each bank monitors the charge on each battery. The ignition battery will reach full charge in about 15 minutes, the trolling motor batteries will take several hours, same one always reaches full charge first.

I like the independent charging because one of the deep cycles always reaches full charge first, even when they are new (although the difference when new is maybe 5 minutes, over the years it grows to about 15 minutes.) If it charged the deep cycle batteries as a single charger it would cut off before one of them reached full charge. This seems like a good system to me, many of the on-board chargers are built this way now. I wonder why they don't do this for RV chargers.

Multi-bank marine chargers have been around for some time. I'm working on a project for a client that has a 48VDC golf cart where the original batteries were replaced with 4 Lifeline GPL-30XT batteries at 12VDC wired in series. Due to the constant voltage nature of a traditional golf cart battery (which has been used to charge the golf cart bank), one of the batteries was getting overly hot and degrading the performance. A local golf cart shop replaced that single Lifeline with a cheapo LIQUID CELL wired in it's place.

The cart still runs, but not for anywhere near the operating time it used to. Nor do the batteries get evenly charged now. My solution, after several conversations with one of the engineers at Pro Charging Systems was to use one of their 4-bank chargers and wire the golf cart for individual battery charging rather than use a 48VDC charger for charging all of the batteries at one time.

This system and it's draw pattern is different than those found in RV use. Golf carts use the full 48VDC for the drive motor but split off either 12VDC or sometimes 24VDC for use in lights, horns, accessories like stereo systems or other add-on items. This one uses a 12VDC Danfoss ref rig. to keep the beer cold!!! (Since RVs use the full capacity of a house battery bank evenly, there is no reason to use individual chargers for the job. Quality, while bank chargers like Magnum inverter/chargers or solar charge controllers like Blue Sky or Morningstar work exceptionally well when the system parameters are properly set.)

That arrangement means that one or 2 of the batteries in the bank get more use and draw than the others. This system then requires additional charge time for those cells that are also being used for aux. power purposes. A standard 48VDC charger will not allow the more heavily serviced battery to attain full charge and leads to premature failure, as they have seen. Ultimately, the more heavily used battery will be rotated (probably on an annual basis) from aux. service to allow all of the batteries in the bank to be fully utilized over the life of the bank.

4 new Lifeline GPL-30XT batteries and a new 4 bank marine charger should solve this problem, along with a few new wiring upgrades and a few combiner/disconnect switches. Time will tell!!!

[Rich W]

Originally Posted by lewster

My solution, after several conversations with one of the engineers at Pro Charging Systems was to use one of their 4-bank chargers and wire the golf cart for individual battery charging rather than use a 48VDC charger for charging all of the batteries at one time.

I've had my 3-bank Pro Charger since about 1997, never had a problem. It's an excellent product and I think they have a lifetime warranty.

I normally pull from both batteries (24 volt) when using the trolling motor and after a full day of fishing the batteries are in need of a charge. If they were charged by the same supply, one would always charge up first, and this can be seen by the charge lights on the Pro Charger. With independent charging the variance between the battery construction is overcome. This may not be necessary for AGMs, not sure., but works great with the flooded cells. The Deka batteries last a long time. My son installed Lifelines for his trolling motor, I'll see if he gets any better service, enough to justify the cost difference.

In any event, I think the golf cart solution you have is exactly what is needed. Sounds like a very nice cart with some great accessories.

I'm not sure if a Pro Charger could be adapted for RV use. Mine weighs 20 lbs. (considerably heavier than the PD I installed) and 2 of the 15 amp connections could be used for the Lifelines. Would the other 15 amp connection work for the AS? I don't know and I wouldn't know how to hook it in anyway. The PD is doing just fine. My beer is cold and the lights are on.

[lewster]

' The PD is doing just fine. My beer is cold and the lights are on.'

When all is said and done………………..that's all that really counts!!!!!!

[sgschwend]

Same as in an EV (electric vehicles) a single battery or a bank of batteries will supply charge or receive charge at the rate based on their internal Thevenin Equivalent Circuits (TEC). When the battery sizes changes in a bank of batteries the charging current for each size battery will be based on that battery's (TEC).

The same could be said for each cell in one battery.

Keep you draw rate low and charge rate low and the heat effects will be minimized and you should be able to handle some variability.

A final point: AGM and Gell are still lead acid batteries, therefore they use the same chemistry. The way AGM and Gell are assembled make them more efficient then the flood batteries. Efficiency differences effects the way they are charged and also their discharge curve. Hooking up a flood battery would just cause them to transfer charge to the flood battery to help it maintain the same Specific Gravity. To some degree this happens in all battery banks.

Good times,,,