Low Water (Batteries)

[FourPoints]

Over the last 2 months I've been noticing my solar has been having trouble getting my two group 27 batteries wired in parallel (but with a switch to isolate them) back to float stage, and they have not been maintaining a charge quite as well. I assumed it was simply because I added the Auto Pilot this year and I've been using it quite a bit, and I've just been running the batteries down lower than I have in the past, resulting in longer recharge times.

After my trip to Block Island over Labor Day with all the motoring we did that shouldn't have been an issue but we still ran the batteries down a bit. I hooked the batteries up to a autozone car battery charger (I don't have a regular shore power charger) and let that sit overnight at the 2amp setting. It still never came up to what I feel was a proper 100% charge, so I knew that there had to be something more going on. After disconnecting the charger last night I checked the water levels and found that Batt 1 was topped off, but Batt 2 was down roughly 2oz of water per cell, although none of the plates were exposed, I'm sure they were when heeled over, especially since I have not rotated my battery boxes yet per your recommendations on orientation.

I added a total of about 14 oz of water to the battery (6 cells, just over 2oz each). I last checked the battery levels this spring, and they were full then.

Should I be concerned that one battery boiled off that much water while the other didn't, and should I be concerned that the water level got that low, even though the plates would have only been exposed at heel, never at rest?

Aside from keeping a closer eye on the water levels of the batteries, is there anything else I should do?

 

[Don S/V ILLusion]

Guessing is never a good method. Get a hydrometer and find out.

 

[jviss]

It's hard to analyze without knowing more about how they are wired. Do you have a schematic or other diagram you can post?

One thought is that one is being boiled, the other disconnecting via the isolation switch when the voltage drops below the setpoint.

Another thought is that one is being used/charged harder than the other.

Are they the same age and make?

 

[FourPoints]

wired parallel, nothing to automatically isolate. Batteries were purchased new from dock master, supposed to be same make, but not One has a Delco label, the other has a Interstate label, he claims they are the same battery otherwise... I couldn't really refuse them for it, though I wanted and tried to, it would have caused too much of a issue...

 

[Bill Roosa]

it is summer and solar does not operate well in the heat. The volts are down and result in lower than normal charging levels. Pretty standard stuff. Always design the panels to output 18+ volts so you don't have this issue, oh yea, you did not design them, stinks to by you.
36 cells as a minimum

 

[Maine Sail]

wired parallel, nothing to automatically isolate. Batteries were purchased new from dock master, supposed to be same make, but not One has a Delco label, the other has a Interstate label, he claims they are the same battery otherwise... I couldn't really refuse them for it, though I wanted and tried to, it would have caused too much of a issue...

They are likely different batteries. Picture??

Also how are you solar charging with the banks isolated by a batt switch? Are you just leaving it on BOTH when you leave the boat??

 

[Rich Stidger]

Ah hah! Two different batteries. Maybe from the same manufacturer, but different in manufacturing date and possibly internal design differences.

Always, always get identical batteries with the same date code. Having batteries "match" is important. Once when I was purchasing four golf cart batteries, I didn't notice that one of them was a different date code. I exchanged that battery to match the other three.

What is probably happening is that the "dry" battery to which you added water, is a stronger battery with a lower internal resistance. This means that when the two batteries are being charged, the charging current splits between the two. But the one with a lower internal impedance will accept more current at a lower charge voltage. So while the charger is waiting for the other battery to become charged and rise in voltage, the dry battery is sucking up the lion's share of the charge current. Current applied in excess of 100% charge level results primarily in heating the battery and boiling off the water.

In short, charging batteries in parallel relies upon the batteries being the same in capacity and type in order for both to be charged properly.

 

[FourPoints]

They are likely different batteries. Picture??

Also how are you solar charging with the banks isolated by a batt switch? Are you just leaving it on BOTH when you leave the boat??

no picture handy, and yes leave it on the both position. Hardly ever switch it to anything else. Aside from the ability to kill power to everything in the event of an emergency, I really only have the switch there so that I can isolate a battery if one goes bad, or disconnect the cables if I ever need to remove one or re-wire something. Otherwise it's just a 2 battery bank.

Ah hah! Two different batteries. Maybe from the same manufacturer, but different in manufacturing date and possibly internal design differences.

Always, always get identical batteries with the same date code. Having batteries "match" is important. Once when I was purchasing four golf cart batteries, I didn't notice that one of them was a different date code. I exchanged that battery to match the other three.

What is probably happening is that the "dry" battery to which you added water, is a stronger battery with a lower internal resistance. This means that when the two batteries are being charged, the charging current splits between the two. But the one with a lower internal impedance will accept more current at a lower charge voltage. So while the charger is waiting for the other battery to become charged and rise in voltage, the dry battery is sucking up the lion's share of the charge current. Current applied in excess of 100% charge level results primarily in heating the battery and boiling off the water.

In short, charging batteries in parallel relies upon the batteries being the same in capacity and type in order for both to be charged properly.

I am familiar with this, and I did try for that, but to refuse to pay for them and tell the dock master he was wrong would have caused lots of friction, and an unknown quantity of animosity, and it just wasn't worth it.

I was worried about exactly this issue initially, however the batteries were installed nearly 2 years ago now, and this is the first time I've seen any issues with them getting out of balance.

 

[Maine Sail]

I was worried about exactly this issue initially, however the batteries were installed nearly 2 years ago now, and this is the first time I've seen any issues with them getting out of balance.

Placing two unmatched batteries into the same permanently parallel wired system can lead to these types of events. What is normal for one battery may not be normal for the other one. They likely have differing internal impedances and might even have differing plate thicknesses etc..

It is one thing to charge different banks in parallel, only when charging them, but it is then best to isolate them for discharging.

This may be normal water use for that battery and normal for the other one but with one of each it is impossible to tell.

 

[walt]

I had the same problem.. Two 12 volt batteries - same model - bought at the same time. Always wired in parallel, always charged that way, always used that way. The wiring to the batteries was also balanced - i.e., the positive comes off the one battery and the neg off the other battery (there is a common circuit diagram for this so I didn’t post it).

These batteries were generally charged with a 20 watt solar panel / Morningstar pwm controller and Id check and one battery would have much higher water use than the other - even though they were supposed to be "identical". Note that once the water level starts to expose the lead on one of the batteries, they now definitely become different and it would seem that this then accelerates the loss in the lower water battery.

I think if you charge with two bat in parallel - no matter how good the setup, you need to check the water more often. After several years, both batteries were toast but the one with the low water (always the same one) was really trashed.

My solution was to just not charge that way, I went to two six volt golf cart batteries (trailer sailor - the two six volt battery bank is the ONLY battery in the boat) and in the few years since I made the change, both six volt batteries take very close to the same water and seem way healthier than the two 12 volt battery setup.

 

[sea_casper]

Placing two unmatched batteries into the same permanently parallel wired system can lead to these types of events. What is normal for one battery may not be normal for the other one. They likely have differing internal impedances and might even have differing plate thicknesses etc..

It is one thing to charge different banks in parallel, only when charging them, but it is then best to isolate them for discharging.

This may be normal water use for that battery and not normal for the other one but with one of each it is impossible to tell.

I do not believe that parallel charge or discharge is a problem with two different battery banks, I have been using two different banks with no problem for two years now, One battery is a wallmart group 29 the other two 6v 220ah's. It seems to me that during charge or disharge each one will contribute or accept the current it can for that voltage, the higher capacity bank will contribute/accept the most current, it will have the lowest internal impedence. I would be most concerned that the two 6v batteries in series are matched for proper charging, as two different acceptance voltages for the same current can confuse the charger which responds to the sum of the two voltages.
Even with identical batteries in parallel, differences in temperature and connectivity will cause differences in charge/discharge currents between the two, but in parallel one cannot discharge/charge to a significant percentage from the other, assumming the same battery chemistry.