Battery / Electrical problem

[Norman_E]

My boat has four batteries, all charged from a single output Mobitronic switch mode charger. The engine start battery, and the two domestic batteries all charge OK, and the engine battery appears properly protected from discharge. The problem is that the bowthruster battery gets very hot indeed when charging, and gasses, requiring frequent topping up. It only charges to about 13 volts measured with a little digital meter, and the domestic batteries give the same reading, indicating that they are in fact all linked in parallel. With the thruster battery disconnected, the other batteries get a better charge.
After disconnecting the bowthruster battery and charging the others for two hours, then turning off the charger and letting the voltage settle I get readings of 13.07 volts from the domestic batteries, 12.67 volts from the engine battery (which is a sealed for life type and has no difficulty cranking the engine).
The bowthruster battery has been disconnected for 24 hours and reads 12.63 volts.
The question is why does the bowthruster battery (which is a Bosch 120 amp hour heavy duty model) get so hot and gas so much that I had to add about 2 litres of water to it before its last charge? It seems to be holding a charge and therefore not dead yet.

 

[steve_cronin]

Because it\'s knackered.....

...classic symptoms of a failing battery. It gets hot on charge, won't hold it's charge and it gasses excessively. Does(do) one or more cell(s) need more water than the rest? If so, that puts the tin hat on it.

Bosch batteries do tend to have a good life (& mine are over 5 years old) but ALL batteries fail in the end and often whilst on charge.

I would also suggest that an off-load multimeter reading of 12.63 after 24hrs doesn't confirm that it is holding charge. What you need is an on-load tester which most battery specialists possess.

An improvement you might also consider is a charger with multiple, individually regulated outputs. There are several about.



Steve Cronin

 

[Norman_E]

Re: Because it\'s knackered.....

Thanks, I had suspected I need an on load tester, but am currently stuck on the boat waiting for the riggers to arrive and replace the backstay. I will try to get it properly tested. All cells needed water, and I suspect that part of the problem was that the battery had been on charge before with far too little electrolyte in it. Looks like I may have to fork out for a new heavy duty battery. OUCH! my wallet hurts!

 

[steve_cronin]

Re: Because it\'s knackered.....

For the bowthruster a high capacity starter battery would be not only cheapest but best as this type is designed for the sort of loads put on it by starter motors, thrusters & windlasses - heavy current demand over a short time and then re-charge. HD house batteries are not - they are made to take repeated long and deep discharges followed by a full and measured re-charge.

So you might not have to spend too much.

Steve Cronin

 

[Pye_End]

Re: Because it\'s knackered.....

Is it possible that the battery is ok but for some reason is getting over charged?

As you say 12.6v doesn't tell you much, but the load test will be more helpful. If the wiring allows you can turn on a light and see what effect it has on the voltage. Can be telling.

Even if battery is fried, is it possible that being non-regulated caused it in the first place?

 

[steve_cronin]

Re: Because it\'s knackered.....

Since these batteries are being charged in parallel and we must assume that the starter battery is of a lesser capacity than the 120Ah of the thruster battery (probably around 80-90Ah) then considering the likely use cycles, one would expect that the starter battery is the more likely candidate to be over-charged.

You're right in that a load like a couple of 55watt car headlamp bulbs would give a better indication of condition but these would need connecting efficiently into the circuit to ensure that any dimming wasn't due to terminal resistances.

As to the battery being "Non-regulated" in it's charging, we are told that it is being charged with a multi-stage charger. However, this would only respond to an average voltage in the four parallelled banks. This is why I suggested a multi outlet charger to enable each bank to receive the appropriate re-plenishment.

Another clue, from the original post is that "...the other batteries charge better with the thruster battery dis-connected." This battery is clearly dragging down the others and should be replaced

Steve Cronin

 

[Norman_E]

Re: Because it\'s knackered.....

Thank you, Your last paragraph mirrors my thoughts, but the rigger doing my new backstay brought down his battery tester. Under load the battery tests OK. We have decided to try it again tomorrow, to see if it is slowly self discharging.

 

[Norman_E]

Re: Because it\'s knackered.....

No it's not. But armed with the load tester I separated the two domestic batteries and tried them and the starter battery individually. One domestic battery absolutely dead! On load it dropped to about 6 volts. Evidently it was causing the charger to run at full output continuously, and heating the bowthruster battery. The latter still shows as OK. THanks to all who replied for the help. It just shows how much expertise there is on the forum, and I have learned not to try and test a linked pair of batteries as if they were a single unit.