Lead acid battery charging.

[ericmark]

I tried to recharge some 7 Ah VRLA batteries some time ago using a Lidi Smart battery charger with good battery in parallel, I was surprised to see a sudden recovery over some 8 hours after they had been on charge for around a week.

So last Friday 8th June I started to charge a 20 Ah VRLA, the parallel battery a 75 Ah leisure battery had just been used with the caravan motor mover so to start with the charge rate was 3.8A quickly dropping to 3A, and then 0.8A by Saturday sitting at 0.1A where they stayed until this Friday. 5:21 am the 20 Ah VRLA started to take charge, then two more peaks as shown, but by 6 pm it had settled down to 0.1A charge again and 13.4 volt, while connected in parallel with the 75 Ah switching off the charger dropped to just over 4 volts. I need 7.3 to 7.5 volt before it will charge without a battery in parallel.

Now I need to consider how long before I accept the battery is duff? The 7 Ah batteries took nearly a week, but once they started then they fully charged, so this is a different pattern. However at 13.4 volt it can remain on charge with no worries.

Has any one used one of the so called smart chargers, like the CTEK MXS 3.8 to revive a VRLA and what results did they get? Or am I on my own being able to monitor what the charger is doing? I am using a MiHome energy monitor on input to battery.

 

[securespark]

Have not tested my CTEK (MXS 5), but it has brought a dead battery back to life. The battery in question was in my Skoda which was off-road for ages while I was unable to drive (still am ) and it had completely failed. In fact, the battery was <2.5V and would not charge. However, I used a hairdryer (it was winter) to bring the voltage up a bit and once it started charging, it worked a treat.
I set it to desulphation and the battery was good as new.

 

[JohnW2]

View attachment 143350So last Friday 8th June I started to charge a 20 Ah VRLA, the parallel battery a 75 Ah leisure battery had just been used with the caravan motor mover so to start with the charge rate was 3.8A quickly dropping to 3A, and then 0.8A by Saturday sitting at 0.1A where they stayed until this Friday. ....

What are the currents you are reporting - the total output of the charger? If so, have you measured the current actually going into the VRLA battery?

Kind Regards, John

 

[ericmark]

The energy meter measures watts, the charger has 4 current levels, 3.8A, 3A, 0.8A and 0.1A, once the voltages have been reached once, this changes to 0.8A and 0.1A only, when volts drop below 12.8 then current goes to 0.8A and when it hits 14.4 it drops again to 0.1A, the resent usage shows a load of spikes as it keeps swapping between 0.8 and 0.1 amp, but the average hourly usage smooths down all the spikes to show gradual rise and fall.

So 2.5W = 0.1A but will not show 2.5W so keeps jumping between 2 and 3 watt, same with 14.5W = 0.8A again some times shown as 14, some times 15 watt.

Without the VRLA battery the leisure battery stays at 0.1 amp and 14.6 volt, with the VRLA it has dropped voltage to 13.3 and as seen on graph it did at one point start switching between 0.8 and 0.1 amp, you are right I should get the Mk8 out and measure actual current to VRLA, however until today that has been nearly zero, disconnecting the leisure battery while it is charging the voltage goes over 15 and it auto switches off.

With last VRLA as soon as it started to charge I was able to remove the croc clip off the leisure battery. I had expected same today, however simply did not happen.

 

[JohnW2]

Without the VRLA battery the leisure battery stays at 0.1 amp and 14.6 volt, with the VRLA it has dropped voltage to 13.3 and as seen on graph it did at one point start switching between 0.8 and 0.1 amp, you are right I should get the Mk8 out and measure actual current to VRLA, however until today that has been nearly zero,

Measuring the actually (VRLA) charging current would seem the most appropriate thing to do.

I cannot help but wonder whether, in what you're describing, the charger has any relevance. If the VRLA battery was going to charge when it's open-circuit voltage was under 7.5V, would it not do so if you simply connected a healthy 12V battery 'in prallel' with it, without using a charger at all?

Kind Regards, John

 

[ericmark]

Yes just connecting a good battery in parrellel would likely charge it just as well, all the charger does is allow me to monitor what is happening.

 

[JohnW2]

Yes just connecting a good battery in parrellel would likely charge it just as well, all the charger does is allow me to monitor what is happening.

Does it, with any certainty? The relevant current could well be that passing from the good battery to the 'bad one', and what comes out of the charger will depend upon the charger's interpretation of what it may find to be a rather confusing situation.

It just seems to me that one could get rid of all the uncertainties, and definitely "know what is happening" by simply connecting an ammeter in series with the 'bad' battery, rather than relying on 'surrogate' measures. Apart from anything else, the efficiency of the charger could well; vary under different charging conditions/stages/whatever - so that the input power to the charger is not necessarily a particularly accurate measure of how much current it is delivering from its output.

Kind Regards, John

 

[JM2]

You could do far worse than stick a regulated 13.8v across it for a week(/s) and see if it/they take significant charge on its/there own.
Or 14.4v current limited (so save boiling off if it actually charges properly).

However, I do suspect you are dealing with extinct batteries.

 

[ericmark]

Unfortunately my variable voltage and current power supply has died, I did use it to charge small batteries, however it seems the cheap Lidi battery charger has an undocumented function, it actually does drop the charge rate to zero.

this morning I found this graph, the 12:00 and 18:00 dips are where I turned it off, but the drop between 17 Jun and 06:00 was the charger, it had not switched off, it had just reduced charge rate to zero with a few spikes, volts showed as 13.1 I removed the leisure battery to see if it would now charge the 20 Ah without it, but the charger switched off showing the VRLA voltage as 5 volt. On switching on again the charge rate was 0.8A for few seconds, then returned to 0.1A, how long it will charge at 0.1A before going to zero again I don't know.

Before switching off this was the graph, wish I had not switched off, as the charge pulses are it seems exactly what is required to revitalise a sulphated battery, I do think your right, and the VRLA is scrap, however I would leave the charger on the leisure battery on charge anyway so it is ready for next time we take caravan out, so may as well leave the VRLA on for another week and see what happens.

As said before seeing the way the 7 Ah batteries recovered I would not have considered leaving on charge for so long, if nothing after 24 hours they went in the bin, the 7 Ah batteries sat doing nothing for around 10 days, then fully recharged.

 

[bernardgreen]

I was surprised to see a sudden recovery over some 8 hours after they had been on charge for around a week.

I had a similar "surprise" but it took longer. It was one of two 10 Ahr batteries that I had lent out. Both were completely flat on return ( circa 9 volts off load )

Charging both from power supplies set at 13.8 volts but limited to 1 amp and left ( forgotten ) for several days. One is now a good battery.
The other was a bloated wreck.. .

With 20 similar batteries to keep in good condition throughout the year and with advice from the manufacturer I built some simple mini chargers with output voltage fixed at 13.8 volts. Current limiting when charging a depleted battery is achieved using the thermal shutdown feature of the regulator. They are on permanent charge fed form a 19 volt DC source. Total current taken from the source is about 250 mA of which 160 mA is that used by the indicator LEDs

 

[ericmark]

The worry about over charging was why I used a battery charger designed for batteries 1.5 to 11 Ah on one of it's setting, however since connected to a leisure battery in parallel it could over charge. The picture above shows charger does actually stop charging, I have found nothing in the instructions to say it does this, if I swap batteries to a 7 Ah instead of 65 Ah then unlikely to over charge, but at moment voltage is 13.2 volt, if I switch off again it will yet again go to 14.4 volt on turning on again. I don't know if the spikes are the switch mode capacitors recharging in the charger or if it actually gives a charging pulse, if latter that would be good. All I can do is watch and see.

I have however moved to a 7Ah battery in parallel thought about it, better safe than sorry, most the time seems to sit at 12.9 volts, at 12.8 it ups charge rate.

 

[ericmark]

The other was a bloated wreck..

I was thinking why should this happen with a regulated voltage within range, then the penny dropped, shorted cell, so swapped parallel battery for 7 Ah, the charger limited to 0.8A in motor cycle mode (1.5 to 11 Ah) and limited energy in a 7 Ah battery.

 

[bernardgreen]

I am certain the bloated battery was due to a failed cell as the heat was concentrated in the middle of the battery.

News of this device ( power supply and float battery charger ) arrived today. http://stontronics.com/products/sbc60

A bit annoying as two months ago I had designed a very similar function into a PCB ( albeit linear regulation ).

 

[ericmark]

I got the Lidi battery chargers cheap, seem to remember around £14 each, however from what I now believe although similar to the CTEK MXS 3.8 they are not the same.

The charger monitors voltage for first 90 seconds to decide if 6 or 12 volt, so although it will charge a battery from 3.8 volt it is the voltage after 90 seconds that matters, if under or over 7.3 ~ 7.5 volt depends if seen as a 6 or 12 volt battery.

Also although at 14.4 volts the charge rate drops to 0.1A, the drop to zero seems to require so time at 0.1A and with a sulphated battery before drops to zero it exceeds 15 volt and auto turns off.

So although it has a section between 7.5 and 10.5 volt where it should pulse charge to help remove the sulphated problem, one would be very lucky to have a small battery which remains under 10.5 volt long enough for the pulsing to work, it did activate on the 75 Ah but not the 7 Ah batteries.

I now have only one battery left needing reviving and likely it will never come up and will be dropped off at the skip site, a new 20 Ah VRLA costs around £40 and a CTEK MXS 3.8 around £52 so not worth replacing the Lidi charger which does most of what I require, in the main able to charge small batteries, the old 12A basic charger is OK for the car battery, but would not want to leave it on a small battery, and even with the larger battery, having a charger that tells you battery is fully charged in an advantage, even when I can use a hydrometer.

The second Lidi charger was bought to re-charge the caravan battery when we had a RCD tripping problem, now cured, and was just to keep things running for that holiday. I did consider using them with an inverter to charge batteries in the car, but tests showed 5 hours driving will likely put 9 Ah of charge into a battery, so hardly worth the effort.

 

[ericmark]

Update, the last 20 Ah battery has never recovered, it seemed to jump up 2 volts at a time, now sitting at around 8 volt open circuit and with a 6W bulb it drops to around 4 volt, but will continue to light the 6W bulb for quite a long time without any signs of getting dimmer, since jumping 2 volts at a time, I assumed cells recovering one at a time. However the two remaining cells do not seem to want to recover, however sitting at 13 volt, which is likely a little on low side.

Battery states on side 13.5 to 13.8 volt in stand by mode, and 14.4 to 15 volt in cycle mode, OK only 0.5 volt under voltage but still under voltage, I do have a small 500 mA 6 VA battery charger with no voltage regulation, I know it raises the voltage to over 15 volt, the only volt meter to hand shows FSD at 16 volt. However after a very short time it does raise the open circuit voltage, so am wondering if the Lidi charger is just too low of a voltage?

Normally after switching charger off then back on, it takes a few hours before it drops from 0.1A to zero, however after playing on Sunday, it was Tuesday morning before it dropped 0.1A to zero. OK only 3.6 Ah (36 hours at 0.1A) but it have clearly taken an extra charge after playing. I thought time for bin.