Parallel Batteries going Kapoot

[Mikebits]

Greetings; We have an ambulance that has four 12v batteries in parallel, and they keep losing their charge. When we park the ambulance we have a switch that supposedly disconnects the batts from the loads and we always put the switch into the off position, yet our batteries keep going dead. Is it possible that we have a bad battery that is draining the other three batteries?

I don't know much about batts, so I am not familiar with battery failure mode, is a load drain failure something that occurs with batteries, and if yes, how do we isolate the bad battery?

Mucho thanks

 

[crutschow]

Yes, you may have a bad battery. If one has a shorted cell it will tend to discharge all the others. If you disconnect all the batteries from each other (one terminal each) you should be able to determine which one has the low voltage.

But if you have one bad battery it's best to change all of them at once with four identical new batteries. Otherwise the mismatch between batteries can cause problems with equalizing the charging.

 

[Mikebits]

Are shorted cells very common? It will be hard to convince my boss. Thanks

 

[crutschow]

Yes, shorted cells are a common source of battery failure. The voltage measurement should show the one with a shorted cell. An auto shop will have a tester that will test each battery under load, which is a more reliable test then just measuring open circuit voltage.

 

[Nigel Goodwin]

With batteries in parallel like that, you should replace them ALL with brand new identical ones - never mix old and new batteries, or different types.

 

[mvs sarma]

using suitable schottkey power diodes, perhaps we can use following schematic . It clearly prevents one battery discharging into other.

I do agree that the power loss is expected across diodes and depending on load the diodes should also be cooled by heat sinks and / or cooling fan.

 

[Nigel Goodwin]

using suitable schottkey power diodes.

And how much are suitable diodes going to cost?, plus you're still losing voltage across them anyway - which you can't really afford.

 

[crutschow]

using suitable schottkey power diodes
, perhaps we can use following schematic . It clearly prevents one battery diacharg in into other.

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You show separate charge and discharge paths for the batteries but that is normally the same wire in a motor vehicle.

 

[WTP Pepper]

You want to tamper with ambulance electrics? Are you mad? Do they operate life critical systems? They certainly do here in the UK and are very tightly regulated for obvious reasons.

I would get them checked out by a pro and replaced if needed by a regulated authority. They will at least have insurance should something like your problem occur.

As you said, you know nothing about batteries and that's where you are in a world of hurt if something happens (or doesn't happen) when you have bodged them after reading details on a website.

 

[JMW]

I recommend you take it to your radio installer. This is not part of the contract so it will cost you. The dealers are usually not capable of working on these systems.
You may want the EMS equipment provider to offer his input as well. 4 batteries is asking for difficulty, 2 large ones, space permitting, is a better way to go. It is probably diesel that should be taken into account. The on board charger must be designed for the batteries in question. Most vehicle alternators are not rated for AGM and may destroy one. GelCels are a lot more forgiving but they too can be destroyed by overcharging.
I don't recommend "isolators" even the ones with Schottky diodes, prevent the regulator from sensing the isolated battery, resulting in either over or under charging. If you must have an EMS bank and start bank you are better off using a 110 VAC inverter with a suitably sized charger to charge the EMS bank when underway. Many of these can charge multiple banks. The batteries must be of the same type though the size may vary. Starting batteries being smaller.
I installed this stuff for 30 + yrs starting with the "orange box" in the 70's.
There are a large number of variables, and with the protocols changing annually, it is difficult to troubleshoot from afar.
Something is not right, if everything is working properly, you shouldn't need to have it charging 24/7 though all of the services on LINY are usually plugged in unless they are on the road.

Let us know.

Jim W