bobsilvers avatar image
bobsilvers asked

How do I Completely Discharge My Batteries Safely?

I purchased 4 off Victron Energy 12v 220Ah Gel Deep Cycle Batteries via Ebay on 2nd April 2017.

The batteries are installed on my narrowboat as leisure batteries.

The batteries performed well initially but, in recent weeks, the performance has deteriorated significantly.

The batteries now hold charge for less than 4 hours. They do not appear to charge fully. It now takes less than one hour to charge them from zero to fully charged.

I have been advised that It seems like the batteries have “memory issues” – they probably need to be taken out, discharged completely and then fully recharged.

Is there a procedure/method statement for completely discharging the battery?

I have been told (by others) that completely discharging the battery can lead to serious damage to the battery. Is there any truth in this?.

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3 Answers
wkirby avatar image
wkirby answered ·

It is true that completely discharging a battery will cause severe and irreversible damage.
Allowing a battery to self discharge and leaving it in this state for extended periods of time also has a detrimental effect on the battery.
Sulphating of the plates is the enemy of lead acid batteries and there is no real recovery from this. There are all sorts of myths, legends and old wives tales of how to breathe new life into a sulphated battery, but there is no real cure I'm afraid.

Here is a link to the book "Energy Unlimited", one of the early chapters explains how to care for lead acid batteries.

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kai avatar image
kai answered ·

I'm only aware of the memory effect being applicable for NiMH/NiCad cells, not lead acids.

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Stuart avatar image Stuart commented ·

Correct. It was only ever NICad cells that suffered memory effect.

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Stuart avatar image
Stuart answered ·

I have to agree 100% with the answers given sadly.

Have you at any point let your batteries fall below 12.10 or lower. The absolute lowest voltage you should ever go to is 11.6. If you have then you will have sulphated the batteries and I am glad to see that WKirby has given the correct advice. You can no more reverse sulfation than hit 70mph on your narrowboat sadly.

Are the batteries stored in a very cold environment at all ? Very low temperatures can affect any type of lead acid batteries. All you can really do is charge the batteries with a decent charger like the victron units and prey. You would need to leave them on charge for a good 48 hours and then isolate and check with a multimeter after a few hours. They should settle down in the high end of 12v.

Have you actually tried charging them with a mains charger or just the solar controller ? It may be you have accidentally changed the charging parameters.

If you have not taken your batteries bellow 12.10 I would expect Victron AGM batteries to last longer than three years. They use good quality batteries ! I appreciate it will be hard to apply a mains charge for all that time living on a narrowboat but that is really your last option. You may well get back some of the capacity but sadly you will never get anywhere near new battery capacity.

I would get yourself a BMV-712 battery monitor so you will be aware if this starts happening again. I know we all wish we could give you better news but this can be the way with any lead-acid battery and no magic powders or liquid can change it.

Have the batteries ever been deep discharged that you are aware of ?

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