Charging a 12v battery from another 12v battery?

Skylark

Skylark

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Not boaty but hoping to tap into the breadth and depth of knowledge from within the forum.

I want to top-up charge a 45 AHr LA car battery, say once per month. Low current drain (alarm fitted).

I do not have access to 240v at the location. I’d prefer not to spend a fortune.

A simple option is to get a second battery, change them over once per month and charge the one removed where 240v is available (back home).

It would be ideal if I could charge one battery from another. Low current and low tech are both desirable attributes for this application.

Many thanks.
 
I’ll be interested in the answers because if they were similar chemistry and less than half a volt difference at the time I’d just bring a much bigger battery each time and a set of jump leads. Obviously that won’t fully charge the battery in situ.

Lots of DC-DC options to limit current but I’m not sure I’d be worried about the current, and I’d still go cheaper with battery charger and inverter if I wanted to leave it closer to fully charged.
 
You could use a dc/dc boost or step-up converter, under a tenner on ebay.
To charge the recipient battery, you'd want to set the volts to 14.4V or so for a few hours.

A solar panel on the dashboard, plugged into the OBD socket is a popular option, but it may make it obvious that you're not coming back soon?
 
RupertW said:
I’ll be interested in the answers because if they were similar chemistry and less than half a volt difference at the time I’d just bring a much bigger battery each time and a set of jump leads. Obviously that won’t fully charge the battery in situ.

Lots of DC-DC options to limit current but I’m not sure I’d be worried about the current, and I’d still go cheaper with battery charger and inverter if I wanted to leave it closer to fully charged.
Click to expand...
The car is 1987 so doesn’t have OBD. It’s in a dark, remote lock-up.
 
Does it need an alarm? I mean, when was the last time you saw someone running out in the middle of the night to investigate why a car alarm is going off? If it’s in a remote lockup then even less chance of an alarm being reacted upon. If it is that ‘special’ the I would look at other ways of securing it or adding a simple tracker even an apple tag or something similar.
 
Daring to cross the forbidden line ....

In RC - we use multi mode programmable chargers .... many are 12v based ... even though they can charge up to 6S and more packs .. (6 x 4.2v : 25.2v)

Ok - unless you spend reasonable money - the charge rate is going to be quite low at around 4 - 5A --- BUT they are smart and will auto shut off once charge is completed.

So for about 30 euros you can buy a 12v powered programmable charger that will cover Lead Acid incl SLA, NiCd, NiMh, LiPo, LiIon, LiFePo ... with display of voltage / amps . time on ... as well as mA delivered ...

We use these at home for charging all sorts of batterys .... as well as on field powered by a car battery or even a sacrificial LiPo etc.

For OP - charge up a car battery at home ... take with the 12v charger to other battery and connect ... only proviso is that donor battery ought to be at least 2x the capacity of the receiving battery ... in OP's case he wants to charge up a 45A/Hr battery ... therefore a 90 - 100A/Hr donor battery is advised ...

But if you are going to hump a battery like that - just swap batterys when needed .. that way you can use a smaller battery ....

Of course a Solar Panel would do the job ... but I read the OP's limitations for that ..
 
Skylark said:
Not boaty but hoping to tap into the breadth and depth of knowledge from within the forum.

I want to top-up charge a 45 AHr LA car battery, say once per month. Low current drain (alarm fitted).

I do not have access to 240v at the location. I’d prefer not to spend a fortune.

A simple option is to get a second battery, change them over once per month and charge the one removed where 240v is available (back home).

It would be ideal if I could charge one battery from another. Low current and low tech are both desirable attributes for this application.

Many thanks.
Click to expand...
Open the lock-up door and run the engine for 30 minutes each time you visit?
 
Its funny actually ... some years ago - I had a Suzuki Vitara in UK as a cheap leave standing - use for few weeks a year car ...

It stood on the Holiday site I had a summer home on ...

I tried all sorts to keep that battery up ... didn't want to diconnect it - as I did not have the radio code ..

Tried Halfords own Car Maintainer Solar panel ... a bigger panel ... but it still ended up needing a jump every time I went back to UK In the end I sold it and accepted to rent a car instead each time.
 
Halford Lithium jump starter if needed. Very portable and useful.

But I'm with those saying disconnect the battery. My boat engine battery goes all winter without a charge.
 
Many thanks to you all, much appreciated.

I’ve only recently moved the car into a lockup. My plan was to remove the battery and one of the wheels for the duration of its winter hibernation. Unfortunately, the Insurer has mandated that it must have a Cat 1 alarm fitted, hence the need to keep the battery topped up.

Good call, @Refueler I also fly model aircraft and have a Turnigy Accucell 6 charger. This can accept a 12v input and can charge 12v LA batteries. I’ll give it a try.

Still happy to read any other suggestions.

Solar isn’t an option. Running the engine for 30 minutes per month is an option but I’m not keen. I’ve wintered cars for many years and don’t normally start them until Spring.
 
The Acc 6 is a good budget charger .... OK - its not exactly a fast charger - but I have two Acc 6 60W and two B6 Mini 80W's .. and they have got me out of a pickle often. Even today ... I pulled my Soviet Mobo up onto the bank ... she has an auto bilge pump served by a 7A/hr SLA and small Solar panel ... the solar hasn;t had any sun for days now and the SLA ran down ...

When I had her pulled up - wanted to pump out the water that had collected .... a quick charge up of a 2200 3S on an ACC 6 ... and 30mins later - I was pumping a good un !!

I've charged my Lawn Tractor batts with them ... even my Volvo XC70 has once had a B6 charger on it !!
 
I would think a 1987 car won't have electronics to get upset by disconnecting the battery, so I'd probably do that.
My experience of leaving old cars with alarms for more than a week is that the alarm goes off when the battery goes down a bit, the alarm then completely wrecks the battery while really annoying the neighbours.
Ideally take the battery home and charge it once a month or something.

The insurance angle is a pain, personally I would take my own risk on theft.
How old is the battery? I've noticed batteries which happily start cars and bikes can have significant signs of old age, in terms of self-discharge and reduced capacity. I've got a car battery which was starting a car OK, it's about 8 years old and only has a fraction of its nominal capacity. It runs some garden lights and an irrigation pump now. A lot of people found their batteries didn't like weeks of neglect in covid.
 
Thanks @B27 I planned to simply remove the battery (and one of the road wheels) for security. Easy then to top-up the battery at home whenever I remember. This plan was scuppered by the Insurer mandating that I get an alarm fitted. Hence my question 😩

It’s not worth the risk of ignoring the insurer.
 
Skylark said:
Not boaty but hoping to tap into the breadth and depth of knowledge from within the forum.

I want to top-up charge a 45 AHr LA car battery, say once per month. Low current drain (alarm fitted).

I do not have access to 240v at the location. I’d prefer not to spend a fortune.

A simple option is to get a second battery, change them over once per month and charge the one removed where 240v is available (back home).

It would be ideal if I could charge one battery from another. Low current and low tech are both desirable attributes for this application.

Many thanks.
Click to expand...
Simply will not work.
 
It must be a car of exceptional value for the insurer to be so plcky. If not, change the insurer.
My Jeep was running it's battery down when left. It was the alarm, so we disconnected the alarm. My horsebox tacho was running down the batteries so l fitted an isolator when it's not being used.
Alternatively, move to a low crime area😃
 
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