Maintaining little used car batteries

[Harry Bloomfield]

My car doesn't see a lot of use, especially in the winter. Now I'm retired, but when I was working I had a company car, so my own car even then had little use. Like many cars these days, it has lots of electronics which in time will discharge the cars battery. It's only 20mA, which is good compared to many cars. So my regime is to once per month, plug it into my smart charger for a couple or so days and bring it to a full charge. My car is always parked in my garage, which has plenty of power available. My charger presently is fixed to a roof beam, with a ciggy lighter plug on its end, so I just plug into the car and push a button on the charger, then disconnect when I get around to it.

I don't like to leave such batteries on charge constantly, even on a smart charger, because on charge they do cause some electrolyte evaporation, which has wrecked batteries before. I have numerous chargers, smart chargers, constant voltage chargers, adjustable constant voltage chargers. The smart ones need to be powered up, then a button pressed to start the charge.

So I'm think of a way to set up a sort of fit and forget charging system, without getting too clever about it. One idea was to employ one of my constant voltage chargers, powered via a plug in time clock, so it comes on for say 15 minutes per day.

Better, might be a system which monitored the off charge battery voltage and if it fell below a set point, would power up the charger for a day.

Anyone got any alternative ideas?

 

[ericmark]

I can only tell you what my Lidi Smart charger does, it has stages, starts at 3.8 amp, then 3 amp, then 0.8 amp then 0.1 amp and in motor cycle mode off. I have never seen it switch off when set to car, (over 12 Ah) but it does in motorcycle (under 12 Ah) mode, and it will auto return only to 0.8 amp mode not 3 or 3.8 amp unless I press a button.

So in motorcycle mode quite safe to leave on 24/7 and on a 7 Ah battery it will show 12.9 volt 99% of the time, there will be the odd spike where it goes to 14.4 volt, but it very quickly falls again to 12.9 volt.

In car mode on a large car battery 75 Ah then it will also hold at 12.9 volt as it seems internal drain over the 0.1 amp charge, but on a smaller battery the voltage does go higher.

I use a MiHome Energenie energy monitor on charger input so from bedroom laptop I can see charge rate in flat two floors down, there is nothing on the smart charger to say 0.1 amp or zero, but because using the energy meter I can see input to charger is zero.

 

[EddieM]

Taking it for a 30 min drive once a week would be better, cars don't like being static for long periods of time.

 

[Stivino]

It sounds like you are trying to re-invent the wheel. Most modern "smart" chargers do exactly what you are looking for.

 

[ptarmigan]

Optimate, have even recovered what i thought were dud batteries.

 

[Harry Bloomfield]

So in motorcycle mode quite safe to leave on 24/7 and on a 7 Ah battery it will show 12.9 volt 99% of the time, there will be the odd spike where it goes to 14.4 volt, but it very quickly falls again to 12.9 volt.

What has got me thinking about it, is that it is a new 77aH battery which I fitted this week. Car is on a Lidl smart charger at the moment, which says battery is fully charged. Battery voltage is 13.03v with charger connected bonnet open and car undisturbed for at least 24hours, so I am not sure whether the 13.03v is just the battery surface charge, or the charger is actually holding that slightly high voltage up - I really ought to check that point.

If the 13.03v is a result of the Lidl charger keeping the battery topped up, then I am happy with that left on permanently. However, the problem with smart chargers is that if mains power is lost, the relay drops out and they disconnect themselves from the battery.

Even worse was an Optimate charger I purchased, which when power was lost, would simply continue running its LED's from the battery it was supposed to be maintaining, rapidly discharge it and wreck the battery, unless spotted in time. It claimed to be the last word in maintaining a motorbike battery.

 

[ericmark]

It does seem not all smart chargers are the same, I bought a second one in an emergency when caravan charger stopped working, but it was a failure as once it switches down, it will not return to 3.8 amp, but the Ctek MSX 3.8 does, Noco and Ring also make what seems the same thing, again with slight variations.

Fact that the Lidi version does more than what the instructions say it does, means the others could also, so hard to compare, what I have found is a battery may absorb 95% of the charge rather fast, but that last 5% takes time, I found a sulphated battery can sit for 2 weeks doing nothing then start to charge, so it would seem a battery needs to be on charge for at least 3 weeks to ensure fully charged, so if charging once a month that means being off charge for one week per month.

I do agree some chargers do seem to over charge, mothers stair lift batteries would last 4 years used every day, 18 months if not used, clearly the charger was over charging the battery. My batteries during lock down have been charged a week at a time then charger moved to next battery, I have around 8 lead acid batteries and two smart chargers, so the AGM tend to have 2 months between top up charge and flooded around one month.

 

[ptarmigan]

Even worse was an Optimate charger I purchased, which when power was lost, would simply continue running its LED's from the battery it was supposed to be maintaining, rapidly discharge it and wreck the battery

Could you not put a diode in to stop this.

 

[ericmark]

I see Lidi have done three versions, one without a volt meter, one at 3.8A and one at 5A I have the 3.8A version.
This was charge for a caravan leisure battery 75 Ah

To start with it had a 7 Ah in parallel as it was too flat to charge, see bump before 06:00 where it was removed, it was like flicking a switch when it did start charging, and by 29 Jun it was nearly fully charged and just after 12:00 it dropped to 0.1 amp, it continued at 0.1 amp and today voltage has risen to 13.4 volt and charger on motor cycle mode, I will guess it would in fullness of time hit 14.4 volts when it will switch off, but over a week and still not fully charged.

 

[ericmark]

Could you not put a diode in to stop this.

No will not work, under 3.8 volt or over 15 volt the charger will assume disconnected and turn off, it turn off if no voltage, so with diode there would be no voltage, so have to let it do what it's designed for.

 

[Harry Bloomfield]

Could you not put a diode in to stop this.

No, a diode would cause 0.7v volts drop, so a battery would not charge at all.

 

[Harry Bloomfield]

No will not work, under 3.8 volt or over 15 volt the charger will assume disconnected and turn off, it turn off if no voltage, so with diode there would be no voltage, so have to let it do what it's designed for.

I think he was discussing the horrible Optimate.

I think above you meant 13.8v. In fact the Lidl chargers will not attempt to charge, unless the voltage across the battery is around >12.05v.

 

[ptarmigan]

I think he was discussing the horrible Optimate

Each to there own as i have maintained 100Ah batteries with it and at the other extreme have a motorbike battery on it now

 

[securespark]

Optimate, have even recovered what i thought were dud batteries.

So did my CTEK charger.

Brilliant kit.

 

[ericmark]

If the voltage is below 3.8 V or above 15 V, the battery will not be charged. The display briefly shows the error message "Err".

This is not whole story however as 7.3 volts or below it sees it as a 6 volt battery. This is where the Ctek worked better as it only charges 12 volt batteries so it can instigate a recovery with the voltage much lower than the Lidi, however the Ctek does not have a built in volt meter so in many ways the Lidi charger works better, well not better, maybe different it the right word. What one needs with a battery recovery is for the charger to fail safe if it does fail, so you can leave it on charge for 3 weeks without going to look and see how it is doing, if you have a simple sulphated battery not too bad, but if you have a sulphated battery with shorted cell, then as the 5 cells start to take charge, they can be over charged due to 6th cell being short circuit, so you want a very low charge rate, the 0.8 amp of the Lidi is better than the 3.8 amp of the Ctek if it transpires you have a shorted cell, but in a caravan returning to 3.8 amp is better than returning to 0.8 amp when battery is loaded.

The computer display goes from 0 - 100 watt to 0 - 20 watt as the output of the charger drops, and the graph would be a very long straight line if continued, so I did not bother with screen shots once down to 0.1 amp or 2 watt input, it only shows in 1 watt increments so the 2 watt becomes 3 watt as the voltage rises, it seems looking at the graph that it is switching between 2 and 3 watt, but that is not really happening it is just the energy monitor can't show 2.5 watt.

What I found with a sulphated battery the voltage will exceed 15 so it switches off, so the 7 Ah in parallel not only raises voltage to over 7.3 volts to charge as 12 volt battery but stops it going over 15 volt which happens when you remove the battery lead and should turn charger off. The larger battery once it starts taking a little charge the parallel battery can be removed, but with smaller batteries it goes over the 15 volt threshold.