Battery State of Charge

jbondy · #1 (**permalink**) 12-29-2008

Went to visit the boat last weekend and found that the house bank (3 3-year-old group 27s) was at 11.4v. The starting battery is at 12.4v. This degree of difference doesn't sound right to me. The only thing connected to the house bank is the HeartLink monitor. I did listen to a bit of radio the last time I was aboard winterizing, but that shouldnt draw much from as large a bank as that, should it?

In prior years, I would visit once/month and connect a 100' extension cord to my onboard charger. When I would visit next, I'd find it unplugged, but clearly only after the bank had come up to charge. This year, I'm more than twice as far from power. I'd have to use at least two cords, and I'm fairly certain one would "walk" if left unattended (the one not attached to the boat). At what point of discharge do I have to start to worry that it might freeze, thus prompt a recharge visit?

TIA

btrayfors · #2 (**permalink**) 12-29-2008

Joel,

Freezing is just one of the things to worry about. I'd be more worried about long-term damage to the batteries.

Any lead acid battery left in a less-than-fully-charged state will deteriorate through sulfation (formation of PbSO4 crystals on the plates) and stratification of the electrolyte, inter alia. This includes brand new batteries on the dealer's shelf.

Exercising and equalization can sometimes help somewhat, but if the lead sulfate crystals are deeply imbedded you may not be able to reverse the damage.

If getting shorepower to the boat is not possible, could you maybe rig a solar panel or two? These could help extend the life of your batteries, though you say they're 3 years old now and with the treatment you describe they probably don't have a lot of capacity left.

If solar panels are impractical, I'd remove the batteries for storage over winter where they can be left on near constant charge with a smart charger. Group 27's aren't all that big -- not like removing 8-D's :-)

Bill

jbondy · #3 (**permalink**) 12-29-2008

I forgot to mention that these are flooded batteries. Is it the case that sulfation occurs only when the state of charge is below a certain point? At what point does this become a concern? If 12.6v is fully charged, is 12.4v a problem, or is it less than that at which point it becomes critical to charge? Does temperature affect the rate of sulfation? I guess I can hope that it is slower at lower temperatures.

In previous years, I don't believe that the state of charge ever got very low, but the combination of the distance to power and the distance from home to the yard are making this more of a challenge this year.

bubb2 · #4 (**permalink**) 12-29-2008

A few questions for you. Do you turn the battery switch off when you leave the boat? There has to be a draw on the low battery somewhere. First year I owned my boat I left the battery switch on because I had a solar panel hooked to one battery and was using the switch to charge both batteries. Every time I would go to the boat the batteries voltage would be down more. I stared cussing the solar panel. I found 2 draws 1) memory on the radio and 2) the little LED lights on my breaker panel. Once I disconnected the breaker panel no more draw and the batteries came right back up. The solar panel was a little maintenance one, by the way.

jbondy · #5 (**permalink**) 12-29-2008

Thanks. One of the reasons for my OP was my concern over the two differing states of charge between the two banks.

I leave the main switch (Blue Seas Dual Circuit) set to "OFF". I have a few items wired to the hot side (house bank) of the main switch, though. One is the bilge pump. Since I have a garboard drain plug (removed for winter storage), the pump is set to "Manual". Two is the stereo memory. I have a switch in that line which I switch off for the winter. The third is the HeartLink 10 battery monitor, which is not switched. The HeartLink is the only known draw. There are no connections to the hot side of the main switch for starter bank.

Based on btrayfors posts above, I've started to think about a small (20w) panel for the house bank and maybe a 10w for the starter. One of the sites I looked at said that one needs a charge controller to prevent overcharging "if the solar panel to battery ratio is greater than .10 W / Ah". My house bank is about 225 AH, so I shouldn't need the controller for a 20w panel. The starter battery might be closer to a problem with a 10w panel, though. I would plan to hard wire the charger to the battery banks for the winter.

What do you mean by, "The solar panel was a little maintenance one, by the way"? Is there a high maintenance kind I should be aware of?

TIA

bubb2 · #6 (**permalink**) 12-29-2008

It is one of these and only puts out 500 milli amps just designed to keep the batteries topped off not to recharge them.

sailingdog · #7 (**permalink**) 12-29-2008

How is the water level in the batteries?? If you haven't checked it recently, you probably should.

If the water level is low, the batteries could be low due to the electrolyte levels not covering the plates fully... in which case, the batteries are likely toast.

I just added a 25 watt solar panel to the boat to keep the batteries topped off on my boat today.

It goes through a charge controller and should be able to keep up with the parasitic loads as well as the self-discharge rate of the wet cells.

chucklesR · #8 (**permalink**) 12-29-2008

Wet cell batteries can self discharge faster than a small panel can put back depending on the size of the bank (it's a percentage thing).

sailingdog · #9 (**permalink**) 12-29-2008

Yup... true that... but in near freezing weather, the self-discharge rate of wet cells drops a good bit...

and I've done a rough estimate of the parasitic loads and self-discharge, and the 25 watt panel should be more than sufficient. Especially since it is going through an MPPT type charge controller.

Quote:

Originally Posted by chucklesR

Wet cell batteries can self discharge faster than a small panel can put back depending on the size of the bank (it's a percentage thing).

jbondy · #10 (**permalink**) 12-29-2008

I did a search on battery discharge rates and got a brain ache!

I'll check water level this weekend. If it is low, do I assume they are dead, or is there a test? Do I refill and then use a hydrometer to test the state of the electrolyte? Or will that tell me nothing about sulfation or whatever the amage is called that occurs when the electrolyte level is left to get too low?

Should I just trash them all and get new AGM batteries? Or golf cart batteries?