12v vehicle charging circuit

[delta9]

Hi guys, looking for some help!

I plan to install some LEDs striplights in back of van...

The circuit has 2 x two way switches (one at back, one at side door)... I've done the circuit it all working.

The lights has its own power from a 12v independant battery, not from the main van battery.

It didn't want to connect to main battery because if light left on by mistake it would drain battery and van won't start.

But I need to steal a 12v supply from the vehicles charging circuit (when engine running) to re-charge the lights battery, first thought was from the alternator to a relay, then to lights +12v battery terminal?

I couldn't think of any other way apart from adding another battery.
Any suggestions? Cheers

 

[Melvyn Best]

Split charging relays have been used for many years to charge leisure batteries in caravans/ motor homes , exactly as you are suggesting

 

[123hotchef]

yes a split charging relay or one of the smart split charge relays they automaticly switch when they sense the charging voltage

 

[delta9]

Thanks guys, that is exactly what I need, cheers

 

[delta9]

The slave battery is a small one, 7aH.

How many amps would it pull during charging? I think I could use a 40a relay instead of 100a used in motorhomes etc as the battery is a lot smaller.

 

[carbon]

Ring make a smartcom charge relay RCT465 that requires only connections to the vehicle battery think it’s rated at about 22 amp and senses when the alternator is charging it does depend on the age of the vehicle. Either that or a dc to dc charger to charge a second battery. I use the smartcom relay in my van to power the phone kit and sat nav and to charge a small sealed battery to run a printer as I didn’t want to modify the existing can bus wiring

 

[delta9]

Thanks carbon for the help, I've ordered the smartcom relay a lot easier to install that, cheers mate

 

[carbon]

The battery I was using was from a alarm system and didn’t experience any problems in possibly 8 years I changed to a larger agm battery and dc to dc charger when I fitted a heater approx two years ago

 

[MattH]

7AH sounds like a SLA batt. Are you using a dedicated charger for this?

You can't let a SLA batt charge directly off the alternator as it won't self limit like an Lead Acid batt and will end up cooking.

 

[voipio]

The ideal solution is a split charge relay connected to the battery via a DC input SLA charger. That way the 7Ah shouldn't get overcharged. The only possible issue is the charger discharging the battery slowly when it is powered off, which can be solved by using a low voltage drop Schottky diode between charger and SLA battery.

 

[MattH]

The ideal solution is a split charge relay connected to the battery via a DC input SLA charger. That way the 7Ah shouldn't get overcharged. The only possible issue is the charger discharging the battery slowly when it is powered off, which can be solved by using a low voltage drop Schottky diode between charger and SLA battery.

Or use a small Lead Acid battery.

 

[delta9]

Hi guys, the battery I got is definitely not the sealed type, so I will use that

The smartcom relay does suggest "no overcharging can take place, so does that mean it would be ok for sealed batteries?

Thanks for all the advice

Edit, carbon used sealed battery so must be all good

 

[carbon]

I’m sure the one I had was no bigger than 3ah may have been 2.3ah from a quick search on the web, it was still working when I removed it and was powering an inkjet printer through a 300w inverter.

 

[brianblock]

I just spliced 2 rows of those ribbon type leds into the interior light that was in the back of van anyway works a treat

 

[Ruffian]

How many led's have you fitted as they drag such little current can probably leave them on for a few days and still start the van.

 

[8ob]

Older vehicles are ok with split chargers but on some of the modern vehicles with smart alternators they wont work.

Bob

Edit, some blurb: https://www.12voltplanet.co.uk/auxiliary-battery-charging-in-vehicles-with-smart-alternators.html

 

[delta9]

I got 288 leds 4x72 led strips, 4.5w each, so 18w total

Good point bob, I will have to check the alternator, possibly I have got a smart alternator, thanjs