I went to fix my father-in-laws stair lift a good few months ago. Problem he had switched it off at mains, however battery failed on the stairs and the manufacturer refused to tell us how to release the brake to wind it back to charging station.

So only option was new batteries, in a bit of a hurry so left the old batteries behind with idea of collecting latter to recharge them, however on return could not find them. So they sat in their hiding place until my father-in-law died.

Most my battery chargers are 8 or 12 amp, clearly too big for 7 Ah batteries, only charger small enough was one got from Lidi, however when I tried to use it, it just switched itself off again, at 15 volts that's what it should do, so battery one was charged by putting in parallel with a 65 Ah battery, so in real terms constant voltage, neither of the 7 Ah batteries seemed to recover to start with, however after some 3 days one seemed to recover.

So now put the two 7 Ah batteries in parallel, still second did not seem to recover, so tried using both to power a cool box, this would have been around 1 pm on the graph and is the reason for the slight rise in charge rate as the already recovered battery recharges.

In real terms two charge rates, 0.8 amp and 0.1 amp, at 14.4 volt the charge rate drops to 0.1 amp, and at 12.8 volt it returns to 0.8 amp the graph records the average charge rate. So the volt meter on the charger would show a slow decay in voltage, then rapid rise until 14.4 volt and slow decay again. the time on 0.8 amp charge rate was very small until 6 pm where it jumped, but not until 10:20 did it really start to recover, by 11:20 pm it was taking the full 0.8 amp and stayed this way until 2:20 when it started to reduce charge rate, so it seems to have accepted around 3 Ah of power before slowly returning to 0.1 amp charge rate. Now 11:45 am sitting at 12.9 volt I would expect to see that gradual rise further. Both batteries can now be charged independently.

This recovery patten was unexpected, the sudden recovery after so long on charge, I would have expected a gradual increase in energy absorbed not the way shown on graph, however without an energy meter with wifi connection to PC one could not have drawn the graph, I was measuring power into the charger but think it is a safe assumption this reflects power out of charger.

Interested in comments.

I admire what you have done, but for the time you have spent, and the battery condition (Most gel batteries do not fully recover after going totally flat) I would have got a new one for less than £17, saved time, grief and money.

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I did fit new batteries, however now I can use old batteries to power things like my FT290R radio.

The Lidi charger was very good with my 75 Ah battery, however it does not do so well with these little batteries, there is a switch off when voltage hits 15 volt, this auto switches off charger when leads are removed. However with these small batteries before the charger could switch to final stage (0.1 amp charging rate) the voltage hits 15 volt and switches off charger.

It states 1.2 Ah to 14 Ah or 14 Ah to 140 Ah two options on charger, and for good batteries yes it works, but for sulphated batteries they really need to be over around 20 Ah for this charger, only way was to connect a good battery in parallel to get it all kick started.

Are they a Yuasa type gel cells ? if so they like a low, slow charge - cycle them a few times and they will come up like new

I don't like charging gel cells with pb chargers, I have a Rc Model charger that does all types with charge and discharge cycles as well as balanced lipo and life charge

https://www.ebay.co.uk/itm/Fusion-LX60B ... SwGzhaGFZd

I use the pb setting at 100ma for 2 hours to pull up low cells then put them on a proper charge cycle

At one point I was an auto electrician, by mid 90's I had moved to industrial, and when I came across the new batteries I realised things have moved on. However as far as lead acid goes, it did not really matter what type, a constant voltage on 6 cells of 13.2 volts would maintain the battery, and one could go to around 13.8 without causing much damage.

However what we all seem to want is speed, so all kinds of methods have been used to speed up the charge rate.

The stage charger has been used with forklift batteries for years, but not much good if you want to use battery while being charged, so the pulse charger arrived on the scene, and I have to admit not sure how it works. Sterling do some really big pulse chargers, but I am more interested in the small units.

There seems to be two reasons to pulse charge, one is to monitor the decay in voltage to adjust charge rate so the battery can be charged really quickly, important for narrow boats and charging with solar panels.

The other seems to be to recover sulphated cells. The Ctek MXS 3.8 seems to have cornered the market, however around £50 for a charger that can only charge at 3.8 amp does seem rather expensive. The big difference is this charger is automatic and can charge a battery from 1.2 Ah to 130 Ah it recharges as quick as it can, which is not that fast at 3.8 amp, but then continues to charge to get that last little bit into the battery.

The Lidi battery charger is a cheap copy of the Ctek, can't really expect a charger costing £14 to be as good as one costing £50, but to be fair, it is not far behind. Both have 4 manual selections off, motor cycle, car, and cold climate. Not tried the Ctek, but the fact you need to press a button to start it charging means any power cut, and charger switches off, so not suitable for a caravan as likely people would not press the button. However the new version does have a volt meter built in, so giving a good indication to battery state.

I first used the Lidi charger to recharge my caravan battery. I had to cheat to start with, 5 minutes with an old 12 amp battery charger to get voltage high enough for the charger to kick in. Ctek will start charging at 2 volt, but Lidi needs 3.8 volt to start charging. It also switches off at 15 volt, this means removing leads from battery auto switches it off.

However it took nearly a week to finally drop into final stage showing fully charged battery, this for a 65 Ah battery. But I also have some small batteries, stair lifts, wheel chairs, and ham radios, the problem is a 12 amp battery charger would destroy these, which is why I bought the Lidi battery charger.

The first version had no volt meter, and I connected both it and a 12 amp battery charger to car, and it has blown what look like 4 surface mount diodes on the board, until then it worked well, slowly bringing a wheel chair battery back to life. However by time I got the replacement that battery had been flat for too long and has now failed. So being care full with replacement and it is disconnected before I try to start a car, and not used with any other charger.

So it was only two weeks ago when I first tried to use it on a small 7 Ah battery. I had honestly expected to clip it on and press the button and it would charge, however this was not the case. To get the battery started I had to connect it in parallel with a good battery, or the volts shot over 15 volts and it switched off.

I had realised output was reflected with input, so plugging it into an energy meter, even though in my garage, I could monitor charge rate with my PC, phone or tablet. So I know at moment charging at 0.1 amp however don't know voltage without going outside.

Had I not been able to monitor and record what it was doing, I would have realised battery had charged, but not how it suddenly went from 0.1 amp to 0.8 amp then back to 0.1 amp some 6 hours latter, although the graph shows a gradual raise in charge rate, this is not really how it happened, the charger can only charge at 0.1, 0.8, 3, or 3.8 amp, and in this case only 0.1 and 0.8 amp. So the volts had risen to 14.4 volts and charge rate had dropped to 0.1 amp, then the volts started to drop, at 12.8 volts it returns to 0.8 amp charge rate, so the volts were going up and down between 12.8 and 14.4 volts the climb rate slowing down each time until it held below 14.4 volts for 3 hours then again oscillating between the two voltages until it stopped dropping below 12.8 so remained charging at 0.1 amp.

Even then it did not stop charging, and over days we have seen the voltage slowly raising. I took battery one off charge when the volts went to 14.2 the other battery was still at 13.5 volts earlier today. What rate of charge would keep volts stable at 13.2 volt which is considered the voltage of a fully charged lead acid I don't know.

In the stair lift the volts were quite high, I was measuring 30 volt (2 x 12 volt batteries) and this is why I think while in daily use the batteries would last years, but when we stopped using it, it over charged the batteries and they only last 2 years. So I switch it off and just switch it on one day a week.

The mobility scooter clearly has a stage charger, so it's batteries are now 15 years old and still able to take it to shops and back around 1.5 mile, but would guess no longer a 25 mile range.

Clearly if you look at valve regulated lead acid batteries it seems they can better recover from lack of charging to over charging once dried out they are rubbish, I have tried opening removing the rubber valves and adding a few drops of pure water to each cell, they do have a little power after, but within a month failed again, so not really worth doing, but sulphated cells do seem to recover as long as lot left too long.

However in the past I have left batteries on charge without monitoring, and without that energy meter and a regulated battery charger one would never know when that 4 hours charge starts and stops.