Maintaining little used car batteries

ericmark · 19 Jul 2020

Harry Bloomfield said:
Me too, it is a lot of potential heat that 12amps can generate in a battery with a shorted cell. If I intended leaving it charging unattended for any length of time, then I would want to make sure the charge current fell to a very low safe value first.

20 years ago I designed and built myself an up to 12v 60amp pulse charger. It's output was only (battery) voltage controlled. I never left that unattended when it was in use, I built it mostly for emergency starting a car with a flat battery - ten minutes charging a flat battery, then you would be able to start the car.

Yesterday evening, with a bit of time to kill - I went searching for information on maintaining such batteries. Varta's battery site seems to be offering some sensible reading, without getting too technical. It agrees with me that batteries should never be left on a permanent charge. It says that recharge requires 1.25 times the amp/hour of the amp/discharge - so if you discharge a battery by 10amp/hour, it will need 10x1.25 = 12.5amp/hour to replace that discharge.

This is what I like with Lidi charger, it does actually turn off once fully charged, and if it turns back on again it only turns on at 0.8 amp which even with shorted cell is unlikely to cause damage. The Ctek MXS 3.8 however will return to 3.8 amp, and the float charger for caravan can bang out 25 amp, I walked past the caravan and did not click on the smell, I looked at local drains first, and I should have known better, the battery was well and truly cooked.

Clearly with shorted cell battery is US anyway, however there is an explosion risk, and risk of damaging the charger, in a boat/caravan/car when in use not really a problem you should as I did smell there is something wrong, but unless you have a hydrogen sulphide detector, only way with unattended battery is very low charge rate.

With the mobility scooter one battery had a shorted cell which in turn killed the other battery, it was left on charge in my father-in-laws garage, and there was no one who would smell if anything went wrong.

I wonder how long those replacement batteries had been in stock before I bought them, got them direct from manufacturer so would hope not long, but clearly were not fully charged and even now 9 watt seems a lot for two AGM batteries if fully charged so maybe still not charged. Although 9 watt = 375 mA so not really much of a charge.

Harry Bloomfield · 19 Jul 2020

They do always say that any new batteries, irrespective of the technology, before being put into service are given a full charge. I think they send them out with only a partial charge. My new Varta E44 needed 12 hours on my Lidl smart charger, to bring up to a full charge, which suggests it only had a 2/3rds charge.

I have recently bought a few 18650 LiPo cells, they seemed to be delivered almost flat.

Harry Bloomfield · 19 Jul 2020

ericmark said:
This is what I like with Lidi charger, it does actually turn off once fully charged, and if it turns back on again it only turns on at 0.8 amp which even with shorted cell is unlikely to cause damage. The Ctek MXS 3.8 however will return to 3.8 amp, and the float charger for caravan can bang out 25 amp, I walked past the caravan and did not click on the smell, I looked at local drains first, and I should have known better, the battery was well and truly cooked.

Clearly with shorted cell battery is US anyway, however there is an explosion risk, and risk of damaging the charger, in a boat/caravan/car when in use not really a problem you should as I did smell there is something wrong, but unless you have a hydrogen sulphide detector, only way with unattended battery is very low charge rate.

That is exactly what happened to a previous leisure battery in my caravan, leaving it on the 13.8 of the caravan's SMPSU - never again will I leave a battery on charge indefinitely. I wouldn't worry too much about the SMPSU, they are all current limited and have thermal cutouts.

ericmark · 19 Jul 2020

Well retested the mobility scooter, it would not do one hill out of house, however it did this time return under its own steam, with a short rest half way up the hill, so it seems batteries were not fully charged, it uses 184 watt when first put back on charge, quickly drops after 7 minutes, bit of a pain as inverter is 150 watt so not quite big enough.

Harry Bloomfield · 19 Jul 2020

ericmark said:
Well retested the mobility scooter, it would not do one hill out of house, however it did this time return under its own steam, with a short rest half way up the hill, so it seems batteries were not fully charged, it uses 184 watt when first put back on charge, quickly drops after 7 minutes, bit of a pain as inverter is 150 watt so not quite big enough.

It's mostly flat around the local area, but downhill beyond - at the top of a hill. It did worry me using the scooter locally, what I would do if the batteries did run out. They are not easy things to have to push. I have a few 12v 30amp/hour gel cells, wrong size for the scooter, but I could botch those on somehow enough to get it home again if I did get stuck. I'm looking forward to this battery analyser arriving, so I can check out these many batteries.

ericmark · 19 Jul 2020

The battery has quickly recharged

and the range is 25 miles, at 4 MPH that means out for more than 5 hours, that is unlikely to happen, the main problem is if we get a puncture. I did change one inner tube, not easy, you need a 13 mm rear and 10 mm front socket 3/8th drive long as 1/2 drive is too think, not some thing easy to do on side of road, so aerosol tin of glue is really only option.

Hill climbing seems about same as electric bike, which sense electric bike 250 watt and scooter 270 watt is to be expected.

Harry Bloomfield · 19 Jul 2020

ericmark said:
Hill climbing seems about same as electric bike, which sense electric bike 250 watt and scooter 270 watt is to be expected.

Our local Dominos have had a young lad on an electric bike, dashing round the village delivering stacks of pizzas. I saw him make several delivery trips on Friday, whilst queuing outside for the F&C's.

Keithmac · 19 Jul 2020

ericmark said:
Instructions say

I think in real terms it is daft to have a charger which can't be left unattended for long periods or overnight, I will guess it is because it can charge at 12 amp which is a fair rate if the battery has a shorted cell, which is why I use the Lidi charger as it is such a low charge rate, it is unlikely to cause a problem even with a shorted cell.

The problem with a stage battery charger is the limited battery size, if battery too small it can be over charged, but the same also applies if the battery is too big, as the current will not drop low enough for it to drop into final stage.

At £48 new the charger you show is quite cheap for a 12 amp charger, they are normally around the £80 so would assume it is not a stage charger, I would have no qualms at using to recharge a car AGM battery for 8 hours if completely discharged, or a flooded for weeks, as the latter can be topped up, but it does need monitoring, at least twice a day, where the likes of the ctek and Lidi chargers don't need that daily monitoring.

GTO is left on an Optimate battery tender, NOT the Maypole battery charger..

Keithmac · 19 Jul 2020

Apples and Oranges.

If I want the car battery maintained over weeks/ months I will use the OPTIMATE, if I want to recover a dead battery I will use the 12amp MAYPOLE.

Harry Bloomfield · 19 Jul 2020

Keithmac said:
Apples and Oranges.

If I want the car battery maintained over weeks/ months I will use the OPTIMATE, if I want to recover a dead battery I will use the 12amp MAYPOLE.

Maypole's description doesn't suggest it is anything but a basic voltage controlled charger, with a high current capability. There is no mention of any ability to recover a battery, that is anything but a little flat.

Keithmac · 19 Jul 2020

It'll recover batteries that are down to sub 3v range without any issues.

Harry Bloomfield · 22 Jul 2020

Well, the BA201 battery analyser arrived this morning and my initial impression is that it is a very handy bit of kit (new toy). It has tested 5 of my gel cells, one of which I thought was not worth keeping and announced them all good. My 10 year old Bosch car battery is also good at 80% capacity, but my caravan's battery is much worse than I imagined at 30%. My new battery in the car shows as fully charged and 100%.

Now to test out the mobility scooter's two batteries. I have not yet managed to work out how to transfer the data to my PC. One thing I don't like is the far too dark smoked display screen - you cannot see its colour screen outside in the daylight, but its fine in the garage.

Mobility scooter batteries show as 100% good.

conny · 1 Aug 2020

Is there a protective film over the screen?

Harry Bloomfield · 1 Aug 2020

conny said:
Is there a protective film over the screen?

I know what you mean - there was, but I removed it. Works great apart from being too dim for use outdoors in daylight.

Maintaining little used car batteries

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