Gel cells in parallel ?

[walker1]

One of my colleauges has assembled a small device that is using 12V from two 12V, 1.2 AH gel cells in parallel.

He claims that it had worked fine for 7 hours in the lab, but today it died twice each after about 30 minutes in the field.

I myself do hold a little suspicion againt that paralleling of the batteries.

I meassured 10.4V and 10.8V on the 2 batteries (disconnected!), after the system died the second time. (I.E. Both prety dead

If paralleled then about 50 mA was flowing between them. I wonder how much would be flowing at 13-14V ?

Can this have worn down the lifetime/capacity of the batteries and/or is battery pralleling simply a "Do Not Do" ?

I do not know how low his supply voltage can be. A pair of shottky diodes might solve the problem by isolating the batteries from each other but not the load. Plenty of those found in switchmodes, so they are easy and fairly cheap to get.

Something similary clever has to be developed for in-circuit charging, too.

Or are there other options ?

 

[felixc]

The best solution is to find a single battery that can fit the application within your space. It is not a joke. If you use two 12V 1.2A-h in parallel, is it possible to use instead two 6V 2.4A-h in series, or a single 12V 2.4 A-h? The packages are quite standardized in these A-H ranges.

I feel it is easier to diagnose cells in series rather than in parallel, and easier to have a circuit that will detect defective cells and prevent detroying them.

Many people say that they do parallel batteries with no problem. It is seen in trucks. The thing is that they have to be of the same brand and same lot number to avoid situations like the one that you are describing.

 

[davidh729]

walker1,
How are you charging the batteries? It is very important that pralleled batteries be in the same state of charge. I would charge them in parallel and always keep them together

Dave

Dave Hyman
iRobot Corp

http://www.irobot.com

 

[walker1]

Davidh729, this could very likely be where the chain came off.

I have seen my colleauge at least once charging the batteries one at a time.

We are now moving towards the diode solution. We still need to find a good way to charge the batteries, though.

During our meassurements today, we used one battery at a time, and each lasted at least 45 minutes. The batteries were simply swaped during a break, so the actual lifetimes are not yet known.