charging small batteries from big ones?

[Alex_Blackwood]

or near discharged LifePo4 ?

Its a matter that may become more important as amount of charge is used out of more modern batterys ..... we have for many years been careful to not exceed 50% use of Lead Acid regardless of form ... so the full battery to discharged battery current has been manageable.
But take a LiFePo4 and discharge that down to its minimum level and it can be a different ball game altogether ...

The OP said 22Ah AGM!!

 

[PaulRainbow]

How much current will a (nearly) fully discharged 22Ah AGM battery take

According to some of the replies on here, a LOT. In real life, a few amps.

 

[PaulRainbow]

or near discharged LifePo4 ?

At 80% DOD LFP will be at 12.8V, the same as a fully charged LA battery (if you're lucky), so no current will flow to the LFP battery.

Its a matter that may become more important as amount of charge is used out of more modern batterys ..... we have for many years been careful to not exceed 50% use of Lead Acid regardless of form ... so the full battery to discharged battery current has been manageable.
But take a LiFePo4 and discharge that down to its minimum level and it can be a different ball game altogether ...

 

[mikefleetwood]

Looks like the DC-DC charger is the best bet. The one on Amazon is 20A, but I think I'd prefer 10A considering the low Ah of the battery and to match the available spare breaker on the switch panel. I'll see if I can find a lower current charger, or buy another 20A breaker.

I have plenty of 2.5mm cable to make up a short flex that will pug into the O/B's battery box.

 

[PaulRainbow]

Looks like the DC-DC charger is the best bet. The one on Amazon is 20A, but I think I'd prefer 10A considering the low Ah of the battery and to match the available spare breaker on the switch panel. I'll see if I can find a lower current charger, or buy another 20A breaker.

I have plenty of 2.5mm cable to make up a short flex that will pug into the O/B's battery box.

Do you have a link to the Amazon charger ?

 

[mikefleetwood]

Do you have a link to the Amazon charger ?

Hi Paul,
this is the one I was looking at: 20A DC-DC charger
But, on searching, this one might suit my needs better: Plug-in DC-DC charger I should be able to plug it into an existing 12V outlet, and fit a plug on the other end to suit my portable battery box.
Thanks, Mike.

 

[Alex_Blackwood]

According to some of the replies on here, a LOT. In real life, a few amps.

Quite!

 

[PaulRainbow]

Hi Paul,
this is the one I was looking at: 20A DC-DC charger

Can't see a way of setting this up and the description is typical cheap smoke and mirrors.

But, on searching, this one might suit my needs better: Plug-in DC-DC charger I should be able to plug it into an existing 12V outlet, and fit a plug on the other end to suit my portable battery box.
Thanks, Mike.

That's designed to maintain a motor bike/jet ski etc battery when not in use, max charge current 2 amps !

You're over thinking this, no doubt as a result of some of the scare mongering replies in this thread. I have fitted more split charging systems to boats than i'd care to count, with zero issues. Seriously, all you need is a VSR, connect one side to the domestic batteries and the other to a breaker, then a socket. Plug the outboard battery into the socket and turn the breaker on.

Victron Energy Cyrix-ct 12/24V 120A Intelligent Battery Combiner - CYR010120011R – BMS Technologies LTD

The realy will close and charge the outboard battery whenever the domestic batteries are on charge, by any means, and close when they are off charge to keep the outboard battery isolated from the domestics.

 

[PaulRainbow]

Quite!

Considering how many boats have split charging, with zero issues, it's a most bizarre thread indeed.

 

[mikefleetwood]

Can't see a way of setting this up and the description is typical cheap smoke and mirrors.

That's designed to maintain a motor bike/jet ski etc battery when not in use, max charge current 2 amps !

You're over thinking this, no doubt as a result of some of the scare mongering replies in this thread. I have fitted more split charging systems to boats than i'd care to count, with zero issues. Seriously, all you need is a VSR, connect one side to the domestic batteries and the other to a breaker, then a socket. Plug the outboard battery into the socket and turn the breaker on.

Victron Energy Cyrix-ct 12/24V 120A Intelligent Battery Combiner - CYR010120011R – BMS Technologies LTD

The realy will close and charge the outboard battery whenever the domestic batteries are on charge, by any means, and close when they are off charge to keep the outboard battery isolated from the domestics.

Thanks Paul,
£35 on Amazon - I'll order one.
I assume it will happily coexist with my Rutland dual charge controller?

 

[PaulRainbow]

Thanks Paul,
£35 on Amazon - I'll order one.
I assume it will happily coexist with my Rutland dual charge controller?

Yep, it doesn't care what's doing the charging, if the domestics are charging and above the set voltage to close, it does so and the charging goes to both batteries.

Good price by the way