Lead acid battery care, how when to charge?

27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
I have in the home a selection of batteries, the theory may say use a smart charger and leave it on the battery, but in practice only have two smart chargers, so I need to move it from battery to battery, and with lock down cars are not being used, so also looking at charging batteries in the cars, mobility scooter, caravan etc. From 7 Ah used to power my radio, to 12 Ah for jump start/air compressor, to 35 Ah mobility scooter those are are VRLA, to 36 Ah in small car sealed, and 90 Ah sealed in larger car and 90 Ah AGM in stop start car I have a load of lead acid batteries.

The car if in use I can forget, but at the moment I have one car which has hardly moved since March, I have recharged the battery around 3 times to date, the 4 x 4 I use a little more, but still not much, again recharged battery once or twice to be on safe side, and the little Honda Jazz has been used the most and also recharged the most, and on Thursday the battery failed. It was not in best condition and I knew every three weeks it needed a charge, so not a big surprise when it failed, so renewed.

So drove around 50 miles with new battery, then put on charge Friday afternoon, I expected either it to take a lot of charging, showing the car was not charging, or very little, but the charger has put in around 15 Ah, not enough to say some thing wrong with car charging system, but a lot more than expected for a new battery. Lights were on so at least 15 amp being used so 50 miles nearly 2 house so if not charging would be nearly completely discharged, and charger showed 12.6 volts when put on battery, so clearly neither fully charged or flat, but no idea how well charged when I got it, since sealed not filled at motorist shop, so could have been sitting around for a long time. Starting engine voltage showed 14.5 volt, seems a little high but within limits.

It seems likely a car alternator will go over static charging voltage as it needs to charge battery in a very short time, I know the open circuit voltage for a two bobbin regulator like the Lucas RB106 or RB108 was 16 volt, it would never hit that with a battery connected.

However the Lidi charger will step down the charge rate with most batteries and hold them at 12.9 volt, as it drops to 12.8 volt it will switch up charge rate until 14.4 volt and then off or down again depending on battery size selected. But take the battery off charge, and it will when put back on can take some time at 14.3 volt before it switches down again, so each time a battery is put back on charge it is held at some time at the higher volts.

At 12 Ah the charger switches off, and at 90 Ah the charger can't exceed 12.9 volt at 0.1 amp, so alternates between 0.1 amp and 0.8 amp. It is the 35 Ah battery which is between the two so 0.1 amp will cause it to rise above 12.8 volt, but it is unlikely to hit 14.4 volt to switch it off.

So if held at 13.8 volt I am not over worried, but mothers old chair lift would hold the battery at 15 volt, and if used daily the batteries would last around 4 to 5 years, but if hardly used, then 2 years was around the maximum as over charged. Father-in-laws chair lift the batteries lasted over 10 years until in error he turned it off, and charge voltage a lot lower at around 13.4 volt. Seen the same with intruder alarms, 0.4 volt can means difference between 2 and 4 years life.

So I know once I take a battery off charge, if not being used, I can leave it for a month before charging again, but leaving it a year will likely cause damage, but where is this sweet point, and does if battery flooded or AGM (VRLA) affect how long, and what about sealed but not AGM? I would think looking at 1 to 3 months between refresher charge, but that is a guess.
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I don't know anything about smart chargers but if you want batteries to last on permanent charge, charge them at 2.2 volts per cell i.e 13.2 volts for a 12 volt battery. It won't fully charge them, or at least not for a long time but it stops them from drying out.

I have had a couple o 110 ah batteries as a stand by supply for my ham radio equipment, I have probably had them for 15 years now and they were secondhand when I bought them. We had a power cut for about 6 hours the other day and the batteries still worked fine.

73 @Peter.N. not far of what the smart charger does, 12.9 volt seems to be standby voltage. As it stands the charger is put onto each battery in turn, what I need is to wire one up to my old FT290R and see what is local.

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Didn't know that.:confused:

You and me both, it caught me out, I have for years used a 12 amp battery charger, likely now some 30 years old, it is so rare that one needs one, also used a regulated power supply set to 13.4 volt, the reports for valve regulated lead acid (VRLA) also called absorbed glass mat (AGM) said these batteries should have a longer live to the flooded cell, and I have just swapped the pair in the mobility scooter after some 15 years of use, one cell when short circuit and over charged the rest. But typically looking at 10 year life.

But the small 7 Ah versions used in stair lifts, and intruder alarms seem to have a problem lasting 2 years, the stair lift if used daily battery life around 4 years, if only used a little, then 2 years, as the battery became over charged, same with an alarm panel, problem is over charging.

So the smart charger switches at around 14.4 volt, then stops or reduces the charge, until it drops to 12.8 volt then switches back on, so if you monitor the charge 98% of the time battery sits at 12.9 volt, then a quick spike as it hits 14.4 then drops again.

However it is a problem with a battery in use, we really do not want our supply voltage varying that much, with the FT290R 2 meter radio the output would remain at 2.5 watt with a very large voltage variation, but with the IC290 the voltage was critical and to get full rated 25 watt you needed 13.8 volt which was the standard alternator output.

Some vehicles even before charge was computer controlled when to 14.2 volt, as it was a compromise as needed to replenish the battery in a limited time, this has been a real problem with narrow boats, where often the stage charger was used, so 14.8 volt until charge rate dropped to 3.8 amp, then voltage dropped to 13.4 volt, idea is to recharge battery within the 6 hours of cruising, it did not really work, and some went to pulse charge where they monitor volt drop end of each pulse to calculate how much charge is required.

There is a problem, can equipment stand 14.8 volt? The new LED bulb for narrow boats rated 10 - 30 volt, but with a transceiver it does not like stage chargers or pulse chargers. Actually if you monitor a standard alternator output that is really a pulse, the voltage regulator is either on or off it works on a average voltage.

However the smart charger I have may be rated 3.8 amp, but it would need to be a very large battery to hold at that output for long, even a discharged 90 Ah will only charge at 3.8 amp for around one hour, then it drops to 3 amp, and this is the problem with lead acid batteries, they take a long time to fully charge. We are looking at days at 0.8 amp to fully charge, and even then longer at 0.1 amp charge rate. The charger I have is zero to 12 Ah and then 12 Ah to 120 Ah rated, set to below 12 Ah it will switch off at 14.4 volt, set to over 12 Ah it will reduce to 0.1 amp.

It is clearly designed to be left on the battery for an extended time, weeks, but I only have two, and one has gone walk about since my daughter tidied up. Once taken off the battery it has to hit 14.4 volt before it returns to maintenance mode, so clearly don't want to move daily between batteries, but a battery sitting on the floor of the flat unused will stay charged for around 3 months, but the one fitted to car, it seems a month is around the limit, and the flooded battery will self discharge faster than an AGM. So around 8 AGM batteries, and 3 flooded. The mobility scooter is biggest problem as battery can only be charged on the scooter, so either an extension lead out to shed, or bring scooter into house. Seems really poor design when batteries have to be fitted to scooter to charge them.

Push bike does not have lead acid, and can be stored in half charged state, so that is not a problem, at moment it is sitting in the bath out of the way, so could charge, although not sure on rules taking a supply into bathroom to charge bike?
Thanks for all that info, very interesting, smart chargers are probably 'smarter' than I gave them credit for. My opinion was coloured by the fact that most early smart chargers didn't work, or at least not on a very flat battery.

Mine are being charged by my 30a radio supply which has a variable voltage feature, I set it at 13.2 volts as that is theoretically the fully charged voltage and that was probably 15 years ago and as I said, they are still going, don't know what the capacity is like now and don't intend to run them flat to find out, but they were running a 2 metre rig with a 100 w linear and would also run the HF rig.

How discharge the Smart charger will work at varies, I am told the Ctek needs around 3 volt, but my cheap Lidi for a 12 volt battery needs 7.5 volt as under that it assumes 6 volt. Also the charger auto turns off at 15 volt it assumes the lead has been removed, so heavy sulphated and it will not charge without some cheating.

My worry with a completely dead battery is the shorted cell, so I use a 7 Ah battery in parallel so maximum it can supply is 7 Ah + 0.8 amp in 12 Ah mode, I have connected the charger to an energy monitor so I can see what it is doing from the PC.

The charge pattern was rather a surprise, nothing for 10 days, then as if some one had flicked a switch, started to charge and seemed to fully recharge, OK only at 0.8 amp but once the graph dropped it seemed to be fully charged, the first two were from a chair lift where the charger had be accidentally switched off. But since I have repeated this for a few batteries both VRLA and flooded.

With 6 volt battery 3.7 volt is lower limit.

Comparing the Lidi smart charger with the Ctek mxs 3.8 although they at first glance they look the same, they are very different, the Ctek will auto restart if there is a power cut the Lidi will not, and the Ctek will return to 3.8 amp if battery voltage drops, the Lidi will only return to 0.8 amp, and the Lidi has a built in volt meter, the Lidi is fixed current 0, 0.1, 0.8, 3, and 3.8 amp, the Ctek uses fixed volts on some steps, there is not a best, as it depends what you are using it for.

I like the fail safe of the Lidi only returning to 0.8 amp output, so less likely to bake a battery with shorted cell.

However 14.4 volt is OK for a few hours, but with a battery just on the edge it could hold it at 14.3 volt for an extended time, and so once off charge, there must be a sweet point at which it should be returned to the charger, too short and over charging and too long and sulphated, the little 7 Ah seem to hold their charge for months, and when I fitted a new battery to my Honda Jazz (40 Ah) I put it on charge, and looking at charge rate and time it took around 9 Ah before fully charged. And it had been on the car for 50 miles (2 hours approx).

Putting flooded batteries on charge not a problem, can top them up, and I know we tried to charge once a month, but AGM/VRLA is another story, I am thinking about 3 months when not on car, and 1 month when on car, the 90 Ah AGM on the Jaguar XE about 1 year old, and took around 20 Ah to recharge after being left for around 2 months.