battery charging issues

[vidalia]

We have four 14 year old wet lead acid 100 AMP hour deep cycle 6 volt batteries wired to create two 12 volt banks. We overwintered the batteries in Michigan on the boat. We charged them using shore power every month and topped off the water if needed. The initial reading at it's lowest was 12.8, but it was usually between 13.5 and 13.8. While charging from shore power both batteries would read 14.2. We would let them equilibrate and recheck and they would both read 13.8. We put the boat in the water this summer and hooked up our 130 watt solar panel, and checked and topped off the water every week. We barely used the batteries (run bilge pump for 10 minutes once a week, and to start the engine, and run instruments for a few hours. The readings have become weird. Bank #1 is reading 14.5, and bank #2 is reading 13 while the solar panel is hooked up. If we unhook the solar panel and let the batteries equilibrate, both batteries read 13. So I bought a digital volt meter, and a temperature compensating hydrometer. The hydrometer 1250-1300 for every cell, except the three I added water to, which without time to equilibrate, of course read low. The voltmeter reads 7.21 and 7.13 (14.34 for both) for bank #1 and 6.21 and 6.22 (12.43 for both) for bank #2. This seems to suggest that the analog volt meter on our boat is reading as much as 0.6 volts higher than the digital voltmeter. Does this mean bank 2 is no longer taking a charge from the solar panel well? When I disconnect the solar panel the charge equilibrates over both banks of batteries, as it should. Have I accidentally disconnected or misconnected something? Yeah I know they are 14 years old and they don't owe me anything, but I am curious about what is going on.

 

[dlochner]

You are correct, the batteries don't owe you anything. Time for some new batteries.

The most accurate readings will be from a decent qualty digital voltmeter with readings taken at the battery while the battery is disconnected and has rested with no charge or discharge after ~24 hours. Analog meters are not that precise and voltage drop over the sense wires does not improve their precision, they are a crude measurement at best.

In general voltage readings are a poor estimate of battery capacity. They can give you a rough idea of the batteries state of charge but tell you nothing about the batteries capacity. Lead Acid batteries lose capacity with each charge and recharge cycle which can't be estimated by voltage alone.

If you did not wait 24 hours with no charge before, at the voltage reading you got were from the surface charge. It is residual and is usually close the float charge voltage, in your case about 13.4v. If the batteries were placed in parallel, the battery with the higher voltage immediately tried to "charge" the battery with lower voltage, thus the voltage in Bank 1 dropped and the voltage in Bank 2 increased. Hard to say why the batteries aren't equal in capacity at this time, could be one bank was used more than the other or it could be due to manufacturing differences.

Anyway you put it, Bank 2 is way past prime and Bank 1 is past its prime. This late in the season I'd wait until next year to replace them and I would remove them from the boat this winter so they don't freeze.

 

[Don S/V ILLusion]

As a corollary to the above, if your charger is as old and/or if you have a battery isolator in the system, one or both should warrant careful inspection for proper charging. The latter(the isolator) being among the most likely reasons for disparate charging.

the last thing you want are new expensive batteries fed by improper charging.

 

[vidalia]

You are correct, the batteries don't owe you anything. Time for some new batteries.

The most accurate readings will be from a decent qualty digital voltmeter with readings taken at the battery while the battery is disconnected and has rested with no charge or discharge after ~24 hours. Analog meters are not that precise and voltage drop over the sense wires does not improve their precision, they are a crude measurement at best.

In general voltage readings are a poor estimate of battery capacity. They can give you a rough idea of the batteries state of charge but tell you nothing about the batteries capacity. Lead Acid batteries lose capacity with each charge and recharge cycle which can't be estimated by voltage alone.

If you did not wait 24 hours with no charge before, at the voltage reading you got were from the surface charge. It is residual and is usually close the float charge voltage, in your case about 13.4v. If the batteries were placed in parallel, the battery with the higher voltage immediately tried to "charge" the battery with lower voltage, thus the voltage in Bank 1 dropped and the voltage in Bank 2 increased. Hard to say why the batteries aren't equal in capacity at this time, could be one bank was used more than the other or it could be due to manufacturing differences.

Anyway you put it, Bank 2 is way past prime and Bank 1 is past its prime. This late in the season I'd wait until next year to replace them and I would remove them from the boat this winter so they don't freeze.

Thanks! I read you only has to wait about 45 minutes to an hour for them to equalize after charging, or turn on something that drew power from them for a few minutes. I know a better test is to charge them, test the equalized voltage then use them to do something challenging like start the engine and then retest the voltage. I don't understand why the solar charger won't charge bank #2, but off the charger, bank #1 seems to top off bank #2. All last summer and all winter we got consistent readings on the analog voltmeter, before the weirdness started happening. I feel like the batteries were going strong and then I broke them!

 

[vidalia]

As a corollary to the above, if your charger is as old and/or if you have a battery isolator in the system, one or both should warrant careful inspection for proper charging. The latter(the isolator) being among the most likely reasons for disparate charging.

the last thing you want are new expensive batteries fed by improper charging.

Thanks! I am pretty sure the banks are run in parallel, and there is no isolator. The fact that bank #1 seems to top off bank #2 when the solar panel is disconnected seems to suggest that to me anyways. But solar controller is from 2006 and I am sure there are better units out there for not much $$$.

 

[Davidasailor26]

You should check on whether there’s some isolator in place. From your description my guess is also that there’s not. At 14 years I think you probably have one bad bank that’s sucking the other one down when there’s no charging source. Since the “good” bank owes you nothing I’d replace them all at the same time to keep them balanced.

 

[dlochner]

Thanks! I read you only has to wait about 45 minutes to an hour for them to equalize after charging, or turn on something that drew power from them for a few minutes. I know a better test is to charge them, test the equalized voltage then use them to do something challenging like start the engine and then retest the voltage. I don't understand why the solar charger won't charge bank #2, but off the charger, bank #1 seems to top off bank #2. All last summer and all winter we got consistent readings on the analog voltmeter, before the weirdness started happening. I feel like the batteries were going strong and then I broke them!

The current provided by the solar charger is determined in part by the acceptance rate of the battery. A better assessment can be made by looking at the current, not voltage, going into the battery. There is an inverse relationship between the charging voltage and charging current. As the internal resistance in the battery increases, the current flowing in will decrease as the voltage rises. Thus, if a voltage is low but the current is high the battery is at less than 100% state of charge. In your case, if the batteries are in parallel one bank is no longer accepting a charge, i.e,. there is low resistance in the bank.

Batteries in parallel are wired incorrectly, one bank will suffer a premature death as it will be used more than the other. It would be really helpful if we saw a wiring diagram or a couple of good photos showing how the batteries are connected.

A common mistake made in wiring batteries in parallel is to have the positive lead to the battery switch and the negative lead come off the same battery. A properly wire 2 battery bank wired in parallel has the positive and negative leads coming off different batteries. I'm going to venture a guess that yours is wired incorrectly and that the "good" bank is still fairly good and the "bad" bank is really dead.

Send us some photos and/or diagrams, we'll get to the bottom of this. Then we can work on upgrading your electrical system.

 

[vidalia]

The current provided by the solar charger is determined in part by the acceptance rate of the battery. A better assessment can be made by looking at the current, not voltage, going into the battery. There is an inverse relationship between the charging voltage and charging current. As the internal resistance in the battery increases, the current flowing in will decrease as the voltage rises. Thus, if a voltage is low but the current is high the battery is at less than 100% state of charge. In your case, if the batteries are in parallel one bank is no longer accepting a charge, i.e,. there is low resistance in the bank.

Batteries in parallel are wired incorrectly, one bank will suffer a premature death as it will be used more than the other. It would be really helpful if we saw a wiring diagram or a couple of good photos showing how the batteries are connected.

A common mistake made in wiring batteries in parallel is to have the positive lead to the battery switch and the negative lead come off the same battery. A properly wire 2 battery bank wired in parallel has the positive and negative leads coming off different batteries. I'm going to venture a guess that yours is wired incorrectly and that the "good" bank is still fairly good and the "bad" bank is really dead.

Send us some photos and/or diagrams, we'll get to the bottom of this. Then we can work on upgrading your electrical system.

I was pretty sure the batteries were wired correctly, and I don't think we have changed anything since last year when they seemed to work as expected (since we left them on the boat). But I agree what you are saying is certainly possible. I will check again next time we are down at the boat. I knew they were old when we decided to leave them on the boat for the winter, so if the died of exposure I wasn't going to feel that bad about it. But since they seemed to survive the winter, I thought they should make it through summer without a problem.

 

[vidalia]

You should check on whether there’s some isolator in place. From your description my guess is also that there’s not. At 14 years I think you probably have one bad bank that’s sucking the other one down when there’s no charging source. Since the “good” bank owes you nothing I’d replace them all at the same time to keep them balanced.

I knew they were old when we decided to leave them on the boat for the winter, so if they died of exposure during the winter I wasn't going to feel that bad about it. But since they seemed to have survived the winter, I thought they should make it through summer without a problem.

 

[Davidasailor26]

I knew they were old when we decided to leave them on the boat for the winter, so if they died of exposure during the winter I wasn't going to feel that bad about it. But since they seemed to have survived the winter, I thought they should make it through summer without a problem.

Ours made this winter without a problem, at 8 years old. The winter before I had load tested them and they both tested as good as new for 20 hour capacity. Then 3 months into this season we noticed voltage dropping faster than it used to. At that point one battery tested totally dead and the other was only 75% health.

 

[Stu Jackson]

14 year old

You really had a good run for your investment.
People tend to forget that batteries are consumable items.

but I am curious about what is going on.

See ^^^.

You should follow Don's advice.

 

[dlochner]

As a corollary to the above, if your charger is as old and/or if you have a battery isolator in the system, one or both should warrant careful inspection for proper charging. The latter(the isolator) being among the most likely reasons for disparate charging.

the last thing you want are new expensive batteries fed by improper charging.

@Don S/V ILLusion just lead you to a rabbit hole, if you survive the trip you will have a good modern DC system that will be reliable, informative, and keep the beer cold while cruising.

 

[Hayden Watson]

At first the two vastly different voltages on the two batteries in parallel was a puzzle to me but it made me think. Lets assume that one of the batteries is failing which will ussually show up as an significant increase in internal resistance. Now you have a current divider where the voltage will be the same on both sides during the charging process but almost of the current will be going through the lower resistance battery. When you are charging a battery it acts like a resistive load in the circuit. The one with higher resistance will not be able to come up to the same charged voltage as the other but will pull down the sensed voltage on the pair. This will cause the chargers to put out more current than the good battery needs but because it is the easy (low resistance) path it gets the current. You end up with a grossly overcharge "good" battery and an undercharged "bad" battery. Once you removed the charge current, they both balance out as the bad battery leaches off the good.
This is why it is always recommended that you replace all batteries in a parallel string at the same time. They need to be matched and yours are not. If they are not matched they need to be isolated.
I have a 14-year old AGM start battery which is still functioning fine. My house bank is a new 560Ah LFP pack. They are only connected for charging through a battery isolator and are playing well together. I have the sense voltage on the AGM and monitor the LFP on the BMS and it is always the lower voltage pack in the setup due to the low internal resistance and the 2-O cables to it.

 

[vidalia]

@Don S/V ILLusion just lead you to a rabbit hole, if you survive the trip you will have a good modern DC system that will be reliable, informative, and keep the beer cold while cruising.

Yes, cold beer is the reason to have solar panels on your boat! Thanks for the warning about the potential problems with an old charge controller. We also have a wind generator, that we did not hook up this summer, but I can't imagine that is a factor, but did not consider the old charge controller.

 

[vidalia]

Ours made this winter without a problem, at 8 years old. The winter before I had load tested them and they both tested as good as new for 20 hour capacity. Then 3 months into this season we noticed voltage dropping faster than it used to. At that point one battery tested totally dead and the other was only 75% health.

Sounds like you are down for new batteries too! Despite there being a problem, I guess we have been lucky we have still been able to start the engine to come back in (my biggest concern).

 

[vidalia]

At first the two vastly different voltages on the two batteries in parallel was a puzzle to me but it made me think. Lets assume that one of the batteries is failing which will ussually show up as an significant increase in internal resistance. Now you have a current divider where the voltage will be the same on both sides during the charging process but almost of the current will be going through the lower resistance battery. When you are charging a battery it acts like a resistive load in the circuit. The one with higher resistance will not be able to come up to the same charged voltage as the other but will pull down the sensed voltage on the pair. This will cause the chargers to put out more current than the good battery needs but because it is the easy (low resistance) path it gets the current. You end up with a grossly overcharge "good" battery and an undercharged "bad" battery. Once you removed the charge current, they both balance out as the bad battery leaches off the good.
This is why it is always recommended that you replace all batteries in a parallel string at the same time. They need to be matched and yours are not. If they are not matched they need to be isolated.
I have a 14-year old AGM start battery which is still functioning fine. My house bank is a new 560Ah LFP pack. They are only connected for charging through a battery isolator and are playing well together. I have the sense voltage on the AGM and monitor the LFP on the BMS and it is always the lower voltage pack in the setup due to the low internal resistance and the 2-O cables to it.

That does seem to make sense. It is big decision to switch to Lithium , since these relatively cheap flooded lead acids have lived so long.

 

[Davidasailor26]

Sounds like you are down for new batteries too! Despite there being a problem, I guess we have been lucky we have still been able to start the engine to come back in (my biggest concern).

We replaced our two AGM house batteries after we tested them and saw the state they were in. Considering also replacing the start and thruster batteries now. They’re the same 8 years old and seem to be not quite as healthy as they were originally.