Are my domestic batteries nearly dead?

davethedog

davethedog

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Morning all and sat here in new town creek watching the rti race and enjoying the sun.

Our boat has 2 x 180ahr Varta domestic batteries ,giving 360ahr total. I charged the batteries last night using the generator to a shown 100% on the nasa bm2 I have.

The nasa bm2 shows that we are drawing between 2 and 10amps depending on freezer cycling etc. so let's say a max of 10 amps.

If that was 10amps and the batteries were at 100% should I have more than 70% total capacity left after 8 hours? That is being over generous on the Amps used too.

I am thinking I may need to replace the domestic batteries.

Dtd
 
Your batts may have a prob but I certainly wouldn’t condemn on this data. In fact I wouldn’t base any decisions on the percentage charge reading of the bm2 - at any state of charge.

Moreover, unless I’m missing something, a few hours on the gen is highly unlikely to have 100% recharged your batts if any significant discharge had occurred. There’s still hope ;)
 
The SOC readings for the BM2 are completely unreliable and as dom said, a short charge from the generator is unlikely to bring them to 100%. You need to properly test the batteries.
 
vyv_cox said:
10 amps seems high. A Danfoss 35 compressor usually takes 3-4 amps, I guess a Danfoss 50 a little more. Something special about yours?
Click to expand...

Our BD50F draws around 7 amps. As others have said, the bar graph on BMs is an indication, not gospel. Fully charged and still at charging voltage, our BMs show 105% until batteries drop to resting voltage. Full charge takes quite a while.
 
davethedog said:
Ok and best wat to fully test them?
Click to expand...
To be sure, probably easiest off the boat - could do one at a time. Get to full charge with a multi-stage charger. Then allow to stabilise for a few hours. Attach a known load, something like a car headlamp bulb, so around 5A. Check the voltage every half-hour with load off until you get to 50% voltage level, around 12.2V
Multiply load ampage by time taken and you have your usable capacity.
Purists will say that the load should change as voltage drops but this will be near enough.
There are articles/videos online showing how to do it automatically but for a one-off probably not worth it. But it might take 9 or 10 hours for each battery if they are in good condition.
 
If you're asking the question, the answer is almost certainly yes.
davethedog said:
Morning all and sat here in new town creek watching the rti race and enjoying the sun.

Our boat has 2 x 180ahr Varta domestic batteries ,giving 360ahr total. I charged the batteries last night using the generator to a shown 100% on the nasa bm2 I have.

The nasa bm2 shows that we are drawing between 2 and 10amps depending on freezer cycling etc. so let's say a max of 10 amps.

If that was 10amps and the batteries were at 100% should I have more than 70% total capacity left after 8 hours? That is being over generous on the Amps used too.

I am thinking I may need to replace the domestic batteries.

Dtd
Click to expand...
 
Here's what I have done so far.

Disconnected both of them from the domestic circuit and then tested them with one of those cca electronic tester thingies. One of which failed and other was fine.

So, have now disconnected the possibly faulty battery and going to see how I goes with just the one battery as I read that if a faulty battery is connected to a good battery it is not good.

Thoughts?

Another thing, unsure of the age of the batteries as we bought the boat 15 months ago and were on her then.
 
davethedog said:
Ok and best wat to fully test them?
Click to expand...

As above, the %age State of Charge on the BM2 is worse than useless. However, it also tells you how many Amps the batteries are drawing when charging. So charge at 14.4V +/- and keep charging until the current drawn falls to about 3 Amps. This might take 6-10 hours. When you have done this, your batteries can be considered fully charged.

Then you can use the BM2 to see how many Ah you use, compared to how far the voltage falls. Might be worth checking over a period of evening to morning, before turning anything on, so the battery voltage is as close to rested as you can get.

Do this whole process a few times, and you should be able to assess where you’re at.

Don’t rely on the BM2 State of charge for anything..... ever! The other facilities are very good :)
 
As said disconnect one battery after fully charging both. Then let the battery be discharged by your normal load. The BM2 should tell you how many Amp hours have been taken out before you get to a nominal discharged battery volts. ie decide you will discharge down to 12v exactly. Note the AH taken out to get to 12v. Then do the same with the other battery. That will give you a comparison of batteries and also might give you some indication of useful amp hours in the batteries.
Even if you only get 50% of the rated 180AH before the battery is flat you are probably doing OK. ie don't throw the battery away. When you can only get 10% out in a useful manner then yes perhaps it is cream crackered.
As said the BM2 %charge is not useful. It at best can only give you a % of what you told it was your total capacity. Your total capacity when new could vary down to 300AH and when old at lot less than that. Don't put too much faith in those gizmos that supposedly tell you how good (capacity) the battery is. Only a long term discharge can do that. IMHO olewill
 
vyv_cox said:
10 amps seems high. A Danfoss 35 compressor usually takes 3-4 amps, I guess a Danfoss 50 a little more. Something special about yours?
Click to expand...

By measurement, I know my Danfoss 35 takes 5.1 amps on start-up and 4.4 amps on normal run with fan engaged - without the fan that drops marginally by about 0.1 amp. However I've replaced the inefficient fan fitted, with a 140mm computer fan.
I'd be highly suspicious of anything but local measurement, and would agree with Vyv that no batteries would fully charge with the short period you ran the generator.
I find modern sealed batteries give about 5 years continuous-use life - old fashioned open cells lasted much longer providing they were regularly serviced.
 
Last edited:
The OP didn't say his fridge was drawing 10a, he said
The nasa bm2 shows that we are drawing between 2 and 10amps depending on freezer cycling etc.
Click to expand...

Mikes test in post #8 will give a reasonable idea of usable capacity, it will also quickly highlight a knackered battery as the voltage will drop pretty quickly.
 
% discharged

To be fair to NASA, all capacity estimates are just estimates. It is provably impossible to calculate the capacity of a battery precisely unless you know the discharge current. Even then it is affected by temperature.

I am willing to believe that there are systems that do better than the BM2. It is still a useful piece of kit. I have checked my current and voltage measurements against laboratory instruments and they are very accurate.
 
Amulet said:
I am willing to believe that there are systems that do better than the BM2. It is still a useful piece of kit. I have checked my current and voltage measurements against laboratory instruments and they are very accurate.
Click to expand...

Victron’ BMV 700 series claims to calc remaining capacity via a proprietary algorithm. Their explanation is a bit voodooish but people seem to find it surprisingly accurate. +1 to your other comments.
 
Voltage and current are easily measured, any battery monitor should be able to do that accurately. SOC requires some calculations to be made and some fare better than others. The monitor needs to be programmed with the battery capacity (total bank capacity). Then it needs to work out when the battery is fully charged. After that it has to keep track of what's being used and what's going in, allowing for some inefficiencies. But they are all up against it a bit with SOC, not least because the ah will change with age.

Current pricing is such that depending where you shop, you can get a Victron monitor for more or less the same, or even less money, than a NASA one.
 
William_H said:
As said the BM2 %charge is not useful. It at best can only give you a % of what you told it was your total capacity.
Click to expand...

I'd say not even that. By a process of iteration and comparison with a Smartgauge, I think I get my actual capacity fairly close. The NASA BM still cant get close to what might be a reasonable estimate of SoC, other than occassionally, and almost certainly by coincidence, (a bit like a stopped clock is correct twice a day :)

I'd say again.... TAKE NO NOTICE WHATSOEVER OF A NASA BM MONITOR'S %AGE STATE OF CHARGE - EVER!!! :)

I'd also repeat that it's other features are excellent!! Amps, Volts, Cumulative Amp Hours - but dont forget to zero the cumulative Amp Hours when you are reasonably sure that your batteries are fully charged, (using tail current values).
 
davethedog said:
Disconnected both of them from the domestic circuit and then tested them with one of those cca electronic tester thingies. One of which failed and other was fine.
Click to expand...

I've never been convinced by "those cca electronic tester thingies" - they always strike me as a way for Kwik Fit or similar to show you why you need a new, and expensive, battery :(

Which tester was it? How did the one that failed, fail??

If you think one has failed, i think the simplest test is to charge to fully charged, (using the tail current on your BM2), then check the Specific Gravities of the cells. This should show you whether one cell, (or more), are significantly different to all the others.

My guess is that you have not been charging to fully charged if you havent been monitoring tail current every time you charge.
 
I’m no electrician, but have always charged for several hours, then measured voltage, then measured again 24 hours later to see if there’s been a significant fall-off. I reckon anything over about 13.2v to be a full charge. If it falls to 12.2v over the 24hrs with no load, then it’s probably time to replace, as long as you’ve checked that the cells are not dry.

A proper sparky/garage may have a load-tester which can help to indicate rapid fall-offs.

Otherwise, I bow to others more technically qualified that I am in these matters...
 
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