Flat Batteries - Will they recharge?

[Guest]

Just popped down to the boat to put the shore power on so the beer in the fridge is nice and cold for a short trip out tommorow evening only to find that the batteries are as dead as the proverbial dodo. We've got two 120amphour batts for domestics and one 120amphour for engine starting and it's the domestic ones that are flat. The engine one's OK, but the domestic's are totally dead, just like Monty Pythons parrot, not a spark of life in them. So I put the charger on them and plan to leave them on charge till tommorrow night but the question is will they recover or have they gone too far? I guess I must have left something switched on, but couldn't find anything on. Could have been the bilge pump? At least in future I'll always turn off the Master Switches. But will they recover? Any thoughts please?

 

[philip_stevens]

Colin,
I did the same thing over winter. I must have left the power on to the TV amplifier, and the solar panel wasn't big enough to combat the drain.

The charger took about three days of 8 hours at a time to bring them both up. Since then, about two months, I have not noticed any difference in capacity due to them going flat - about 6 volts!

Give it a try. You've nothing to lose at that stage.

regards,
Philip

regards,
Philip

 

[david_steward]

re: Don\'t turn them off at sea.nm

nm

Dave S

 

[byron]

I'm certainly no expert but recalling the old days. When one bought a battery it was taken from the shelf uncharged and the dealer would charge it up prior to selling it. Surely this would indicate that your batteries should charge up. Also remember old style mobile phone and video batteries. They actually reccomended a complete discharge from time to time. If your domestics are deep discharge they will take a real long time to charge up fully and best done on trickle.

©2001

 

[Guest]

How old are the batteries and what sort of usage have they had? If old you could always take them to a commercial vehicle battery dealer and get them to charge and drop test them. It is possible that one battery is down and dragging the other one with it!!! Try charging up see what happens but I would be inclined to get a drop test done if for no other reason than peace of mind!!

Pete

 

[gunnarsilins]

Don´t compare with mobile phone and videobatteries, they are an entirely different animal. In the boat you have lead acid batteries, mobile phone batteries are cadmium or metal hybrid batteries. The latter, especially cadmium types should be totally discharged now and then.
But lead acid batteries should never be totally discharged, for longest life, avoid discharging below 2/3 of capacity. This does not mean that a totally discharged lead acid battery have passed away, but it has probably taken quite a lot of abuse. Just recharge it and see if it lives up its capacity.

 

[byron]

Thanks! My profile does mention my giving bad advice. You have however jogged my memory. Some time ago my Batteries weren't as good as they should have been someone reccomended tablets from Halfords, these I bought and they reinvigorated my batteries no end.

©2001

 

[colinroybarrett]

Colin,

Don’t expect much to have happened in 24 hours, unless you have one hell of a battery charger?

If we leave the engine battery out of this equation, you still have over 240 A/Hours to put back in.

With the average “Halfords” car battery charger (despite what it says on the label) after an initial splurge at 6+ amps (maybe). As soon as the voltage of the batteries rises much above 12.2-12.8volts the charge current will have dropped to 2-3 amps at the most. If we allow for a bit of “chemical hysteresis”, then we have:-

(120x2) x1.414 = 340 A/Hour needed to recharge fully.

340 / 3amps = 113 hours.

This is the best part of 5 days to recharge both batteries, assuming all else remains good (temperature and shore power stability etc).

Unless your charger really kicks out more than 5amps I would say you can quite happily leave it going all week. Check the volts don't rise much above 14.5 @ sea temperature 13.8 @ 20deg or 13.5 above and you should be OK.

Do check the electrolyte level, but don’t fully top ‘em up (unless really down) until they’ve been on charge –slight gassing- for a while, as you will find the level will overflow from flat to fully charged.

Good luck!

 

[trevor]

Colin,
I suggest you remove the battery from your boat, I know its heavy!
Take it home and charge it at 3-5 amps for 24 hrs then discharge it until the voltage reaches about 12v then recharge it for about 12 hours at trickle charge repeat this twice then chage at about 3 amps for about 48hrs.
If the battery is ok it should then provide full capacity.
To discharge the battery use a headlamp bulb 24-36 watt for a 12v battery.
The princple is to exercise the charge/discharge cycle, you cannot do this satisfactorily on board.
Good luck

 

[oldharry]

The simple answer is yes, if they were in good shape to start with and haven't been flattened previously, then if they accept a charge they will hold it. Forget about the advice to flatten batteries occasionally - thats for NiCads etc. Lead acid batteries do not like being flattened, and each time you do it, you will reduce capacity by possibly as much as 20% - depending how flat is flat, and how long they are left. In other words they will continue to work quite OK, but your 100 ah will now only hold 80 ah or thereabouts, and would go down quicker next time. 'Deep cycle', 'leisure' or 'traction' batteries fare rather better than ordinary car batteries when abused in this way, because of the method of construction.

The big 'IF' is whether they are so badly damaged that they will no longer accept a charge. At that point your expensive batteries are scrap.

'Revitalising tablets' - if only there was such a thing - for me, as well as my batteries! Yes like any 'patent mecicine' they might make a little difference for alittle while, but tired old batteries are - like their human counterparts - just that!

Oh, and re chargers: the average charger as sold by Hlafords for 'car batteries' are a waste of money. They only bring batteries up to around 85 - 90% charge. To do the job properly you need an electronically controlled pulse charger - only worth it if you are gpoing to be charging batteries this way regularly as they are expensive.

For occasional emergency use go down to your local Argos store and get a 'Gunson Automatic' charger for £20. Although not a proper pulse charger, they will bring batteries up to full charge given a bit if time, and has a 'float' switch, which allows you to leave it permanently connected safely all winter keeping your expensive batteries in tip top form.