Understanding my batteries

[robinm 2]

I am having trouble working out whether my batteries are stuffed or not.

 

I have two 5 year old 85ah lead acid batteries which due to mangled cell caps hadn't been topped up in some time, I don't know how long as I inherited them with an unknown history. With the aid of much ingenuity and elbow grease I removed the caps and topped up the cells to revive the batteries. Each battery took almost a litre of demineralised water!

 

I used my ctek mxs10 to recondition the batteries, the program successfully completed, then I left them to fully charge. On removing the charger each battery measured 13.45v on the multimeter. After about an hour this dropped to 12.6v and on returning to the boat 10 hours later they had settled down to 12.4v

 

Are these batteries worth keeping or is it time for new ones?

[Bogan 8]

They're 12v batteries. They will settle to a little over 12v - so there's nothing wrong there.

 

They could well be stuffed. The test of that is how much the voltage drops when the battery is asked to deliver a reasonable load and what their useful capacity is now (which is hard to measure without actually flattening them - which is not something you want to do).

[erice 732]

12.4 is pretty low with no load

 

got a car stereo on the boat?

 

run that for 30min at a reasonable volume

 

with no solar

 

if the bat voltage drops below 12v after that

 

they're probably on their last legs

[ex TL systems 63]

I thought good batteries should hold 12.6 , I have had 3 yr old batteries that sat idle and were still on 12.6 v after a month or more.

[Island Time 1,230]

12.4 v after a few hours is about 75% charged.

Sounds like they are pretty tired. Do you have a hydrometer? When fully charged and no longer accepting charge, the specific gravity of the electrolyte will tell you more, or, better, a proper capacity test. That takes 20 odd hours with a specific load.

[Island Time 1,230]

Oh, and a good,fully charged battery is normally 12.8 volts at rest with no load.

[robinm 2]

Thanks guys, I will do the stereo test and see where I get to. I don't have a hydrometer but I guess the stereo is a fairly practical test. I'll let you know

[wheels 543]

It is possible, in fact most likely the Plates have been damaged.

You sure they are only 85Ahr? that is very small. Like a motorbike Battery.

There are several things that need to be done. Firstly, forget the Ctek Battery charger for bulk charging. You need to thump some current into these things. If you can monitor the charge and ensure they don't get too hot, then a charge with a current of 50% of their rated capacity is needed. If not possible, then a charge current of 20% at the least. Once the Battery has been brought up to full charge, then you need to run an equalization charge. The Ctek may have this facility and it should be capable of supplying enough current. It needs to run for a good 8hrs and you need to monitor the water level and keep topping up as/if the level drops. Then leave the battery resting for 24hrs not connected to anything. After 24hrs, Measure the Voltage and if it is below 12.6V, then they are not good. If you only get 12.4, then they are stuffed. They will retain very little capacity.
If you can get a proper tester from an Autosparky, it will tell you in an instant if the batteries are any good. Or if not, then load test.
A Load Test is a little difficult to do. You need to have a known load or Current draw, so you can do some Math. For instance, 85Hr, will give you 85A over 1hr. Although don't ever test a battery at it's full capacity rating as you will Damage it and maybe yourself. On a Battery, there should be written a C rating number. The number maybe 10 or 20 or something like that. It means the Full capacity can be discharged over that time. So if it was 10 for instance, that means the 85A can be discharged at a rate of 8.5A over 10hrs which is a total of the 85A.
CCA could also be another number. That is different and means Cold Cranking Amps and means for a very short duration, maybe 30seconds at most, the battery can deliver that specified current. This also means the batteries are Start Batteries and not deep cycle.

[too_tall 15]

Wheels. 85ah is a large car battery size in a deep cycle. That's unlike you! Did your coffee not work today?

 

But yes, sounds like the batteries are bricks not batteries..

[wheels 543]

You are quite right TT. I had a brain fade with that figure for some reason. I was drinking my coffee, but it had not taken affect at that point.

[robinm 2]

Thanks wheels, yes a large car battery sized deep

cycle battery is what they are. They settled at 12.4v and dropped to 12.2 after using the stereo for 30 mins this morning. Sounds like they are stuffed

[Island Time 1,230]

Yep. Weights, not batteries. Virtually no remaining capacity...

[too_tall 15]

I had a set of batteries which would charge very nicely to 14.4 and rest at 12,9. They would hold that voltage for an hour with a 20 amp load ( was a 400ah bank ) and after taking around 60ah from them they would still be sitting at 12.4V under load ( bouncing back to 12.6 unloaded within 5 seconds and stay there indefinitely ). Then the next hour would drop the voltage to 10.5 with the load on them. My carbon pile tester would happily extract 500 amps from each 6 volt cell with only a 1.2 v loss. Thats significant current discharge capability!. 

 

I don't know what was happening in there, but they are now consigned to some islanders solar installation where apparently they perform very well he informs me.

 

To suddenly collapse to nothing from what was showing to be a reasonable ( although not great ) load test confused me, its not what I had seen happening in batteries before. 

 

Remember that when testing batteries, a known load over a measured time period is the best way to achieve a good knowledge of the batteries. If you have a battery monitor which counts the coulombs your going to know just how many AH you have taken from the battery and that makes it a lot easier. Don't stress if the voltage drops to 12V whilst under load - its what it recovers to which tells the true state of charge.

[too_tall 15]

P.S. I doubt that battery of yours could start a motor - so if your planning a shared duty installation or have a shared duty installation, be very wary!.

[robinm 2]

Thankfully I have a good separate start battery which behaves like a gem. Maintenance free of course. We are planning a christmas holiday trip to Durville and Abel Tasman so as long as I can cross the strait with my VHF and nav lights working (LED) and leave an anchor light on overnight I don't really mind. I think the budget may be best spent on a Rocna. I don't have an electric fridge or windlass, but the electric head pump is a concern. Ahhh what to do, what to do...

[Jon 360]

but the electric head pump is a concern. Ahhh what to do, what to do...

If you think this is a problem then tell your crew they can only use the head when your under motor.

[robinm 2]

You can buy a anchor for 1/2 the price (or less) that will work the same or better.

What would you recommend for a Lotus 9.2 in place of a Rocna 10 KM?

[robinm 2]

Awesome, thanks KM

[Marshy 30]

New stink boat has a Manson Supreme, have been cruising with people who had them for years and always heard good things but never bothered to change.

Went out last Tuesday and anchored in Motuihe channel - 15knot northerly and a bit of chop. Anchor went down, probably not enough chain to be honest, but the thing bit straight away - was seriously supprised. Spent the arvo tinkering around for fish and anchored probably 6 times in different depths and bottom composition, and never had a problem at all. 

Thats a vindex 350 with plenty of windage, and a 25lb Supreme... Pretty impressed.