I have an old battery in the garage which is old, and I have a new battery on my banged up car which looks likely to end up going for scrap but has a new battery, only 7 months old, which I'd like to swap out for the older one (if it accepts a charge) so that I can fit the 7-month-old one into my new car when it's ready for collection from the dealer who is currently addressing a few points as agreed before he'll release the car to me.

The new car has what appears to be an older battery and damp on the top (might just be water if they jetted the engine bay) which could indicate a battery problem. Chances are that the old car might be taken away before I can collect the car so I need to source a battery charger or a 2nd hand battery before that happens.

As I know you don't believe me on such things there's loads of info on the internet about this.  Why are you bothered about the health of the battery going in the scrap car.  If you want to put some charge in the old battery using your charger then do so I'm sure you know enough not to do any damage that would matter for the very short time of the car being collected.

 

I will dare to suggest that in my experience of old, flat/"dead" batteries using a 2, 3 or 4-amp or slow charge setting will be better than something like 12 amp on normal or quick charge which always seems to kill them if they're already in poor health - but that's just my totally unprofessional, non-expert (in any way) experience over a number of years with neighbours' and friends' car batteries.

 

Then for your new battery in your new to you car buy yourself a 3 or 4-amp charger and maintainer suited to AGM they're not expensive I've seen people report success using £15 battery charger and maintainer from Aldi - only in case you accidentally drain your new battery too low for the computers or yourself as otherwise of course the car's alternator means you never need a battery charger. 😉

 

Edited 14 May, 20233 yr by nta16
missing word

I bought (well,  received as a present tbh) a cheap battery charger recently.

It has 4 modes including AGM. On both wet and AGM  batteries I tried it on, both of which were low on charge, the difference between normal and AGM appeared to be a slightly lower voltage for the AGM mode.

This aligns with my understanding that overcharging is the main risk with completely sealed battery technologies.

 

Probably also what's really behind the whole "only charges to 80%" thing in active service, but dressed up by marketing as 'micro hybrid, energy saving'.

Thanks for the input, I was asking because I needed to make a quick decision about whether I purchase a new but second-hand battery charger for normal batteries (£15 on Gumtree) still sealed in its box, or to get a second-hand battery. The cheapest price from a scrapyard for a battery was £40, decision made, charger it was then.

 

Checking, like you said on the internet, did eventually reveal that a AGM battery should only be charged to a maximum voltage of 14.4V otherwise there is a real risk of overheating the battery and possibly exploding as a result, unlikely but possible all the same. Normal batteries are charged upto 14.8V, and the chargers designed specifically for AGM type batteries (Start/Stop) are intelligent and cut off the charging once the battery voltage reaches the 14.4V but they are almost double the price of normal chargers.

 

Opening up the charger reveals an adjustment potentiometer in the circuit that I think will allow me to tweak the charger to stop charging once the back EMF reaches 14.4V anyway, so getting myself an intelligent charger on the cheap for future use.

 

The battery is on charge and seems to be responding nicely, rising from 8V upto 13.9V now and the battery is nice and cold so no sign of it getting warm, let alone overheating. I'm keeping an eye on it every 90 minutes so will be able to detect any appreciable temperature rise before damage is done.

 

I'm confident now that I can use this battery to start the car if I leave it on charge until just before Copart come to collect it on their transporter, that it will start the engine, I can always give it boost with jump leads from the saved battery if needed.

From my neighbour's Ring RSC612 (12 amp battery charger / tester). -

 

An article of t'web, selling sum'at so 'oo knows. -"Fact is, AGM batteries must be charged and maintained differently to standard lead-acid batteries.  A simple rule of thumb: AGM batteries hold a higher voltage and therefore need to be charged and maintained at a higher voltage." - https://theshopmag.com/features/how-get-most-out-agm-batteries/

 

With the following I am not suggesting anyone needs to or should follow what I done, perhaps I'm a maverick that likes to live dangerously.  I topped-up fully charged the new AGM bought before fitting it to my wife's Fabia Mk3, I used one or both (I forget) of my 20+ and 30+ year old standard chargers, perhaps by doing this I might have shorted the life of the battery from 10 to 9 years or 4 to 5 as VW would have it, who knows, only time will tell.  That if the car's alternator doesn't play up or the battery isn't abused by the owner and users.

 

The internet and the answers you can get is very democratic all answers there to suit everyone The Donald luvs it. 😄

 

Edited 14 May, 20233 yr by nta16

12 minutes ago, nta16 said:

From my neighbour's Ring RSC612 (12 amp battery charger / tester). -

 

An article of t'web, selling sum'at so 'oo knows. -"Fact is, AGM batteries must be charged and maintained differently to standard lead-acid batteries.  A simple rule of thumb: AGM batteries hold a higher voltage and therefore need to be charged and maintained at a higher voltage." - https://theshopmag.com/features/how-get-most-out-agm-batteries/

 

With the following I am not suggesting anyone needs to or should follow what I done, perhaps I'm a maverick that likes to live dangerously.  I topped-up fully charged the new AGM bought before fitting it to my wife's Fabia Mk3, I used one or both (I forget) of my 20+ and 30+ year old standard chargers, perhaps by doing this I might have shorted the life of the battery from 10 to 9 years or 4 to 5 as VW would have it, who knows, only time will tell.  That if the car's alternator doesn't play up or the battery isn't abused by the owner and users.

 

The internet and the answers you can get is very democratic all answers there to suit everyone The Donald luvs it. 😄

 

Yep, if you look long enough you can always find differing information on the internet, the information I saw came from a battery manufacturer, but either way, I'm fine with the standard battery charger as the car does not stand around long enough to drain the battery, it is in use almost daily, so it receives regular top-ups. If it was not being used for extended periods of time then it would benefit from a battery maintainer or slow trickle charger which will charge a few milliamps just to keep it topped up and feed the onboard ECU's.

I think much of the confusion stems from the fact that when not full, AGM or EFB batteries MAY be charged harder and faster than normal lead acids. People falsely extrapolate from that to thinking they must be charged at a higher voltage. 

Edited 14 May, 20233 yr by Breezy_Pete
More emphasis on fullness/not

23 hours ago, Graham Butcher said:

The battery is on charge and seems to be responding nicely, rising from 8V upto 13.9V now and the battery is nice and cold so no sign of it getting warm, let alone overheating. I'm keeping an eye on it every 90 minutes so will be able to detect any appreciable temperature rise before damage is done.

If the battery doesn't get a little warm from charging then it might not be charging enough, but if you only want to the battery to start the car and get the car on to whatever's taking it away you only need sufficient for that.

 

8v wasn't that low compared with the last two batteries I saw but some modern chargers need fooling if the battery is below 10.5V, others at 7-something volts IIRC (which is always doubtful).

 

I think you might be a bit over-cautious with the charger's maximum voltage for the job in hand but I'm sure you'll get there, I'd be low and slow on the amps and have some reserve in the battery in case for whatever reason the car doesn't start well or there's a shuffle to load it or they turn up a day or two late for collection, Sod's Law always seems to apply when least convenient.

 

1 hour ago, nta16 said:

I think you might be a bit over-cautious with the charger's maximum voltage for the job in hand but I'm sure you'll get there, I'd be low and slow on the amps and have some reserve in the battery in case for whatever reason the car doesn't start well or there's a shuffle to load it or they turn up a day or two late for collection, Sod's Law always seems to apply when least convenient.

Can't control the amps, it is a 4amp charger and the only thing that can control the amps is the BEMF of the battery itself which as the voltage rises, reduces the current down to 1.1amps, a little more than a trickle charge, but I think the battery hold up for the purposes of getting the car loaded onto the transporter OK as it held 12.5V overnight without being on charge, so although it did prove unreliable in normal service, it should be OK for what I need. I'll only fit the battery on the day that are going to collect the car so the ECU's don't drain it too much.