Charging options for motorcycle battery.

[iClick]

I have a small Ammo case battery build which contains a 18a/h lithium motor cycle battery that needs charging while I’m out camping.

If I connect it in parallel via Anderson plug to my larger 310a battery will it charge?

 

[Hedges]

Not a good thing to do unless you want to experience your avatar picture again.

A lithium battery at low SoC connected to one at high SoC might draw massive current.
One option would be to determine max voltage of 310 Ah battery and min voltage of 10 Ah battery.
Choose a target charging current.
Calculate resistor ohms and watts to limit the current.
Crude but might be effective.

Better, a buck converter designed for constant current.

 

[robbob2112]

Dc-dc charger...victron Orion or any other brand that you can control the charge profile and C rate with

 

[rci]

I hope I'm not misunderstanding your intent but here's what I currently do.....
I bought a "Battery Tender" brand trickle charger and 'terminal harness' for the motorcycle battery on Amazon:

Amazon.com

Then, to make it easy I got a "Terminal Harness Accessory Cable" and connected it to my Motorcycle battery so that when I want to ride I just disconnect the Harness from the battery tender and re-connect it when I bring it home (I simply leave the harness connector sticking out the back of the seat then re-connect it when needed).:

Amazon.com

I'm sure there are cheaper substitutes, I just like the quality of the "Battery Tender" Brand.
If this isn't something you can use I apologize in advance .... rci

 

[robbob2112]

So, the trickle charger you link plugs in. Are you intending to plug it into an inverter or grid power?

If so that should work. In general it is best to just charge the lfp battery and leave it alone, not trickle charge constantly. There is no need and the lfp isn't happy with it.

A dc-dc charger connects directly to the large battery and sends a regulated and current limited charge to the small battery. Once the small battery is charged it cuts off until the SOC of the small battery is low enough it charges again. What it doesn't do is leave a constant voltage to trickle like a lead acid battery likes.

 

[iClick]

Not a good thing to do unless you want to experience your avatar picture again.

Haha - yes that’s a look best avoided!

A lithium battery at low SoC connected to one at high SoC might draw massive current.
One option would be to determine max voltage of 310 Ah battery and min voltage of 10 Ah battery.
Choose a target charging current.
Calculate resistor ohms and watts to limit the current.
Crude but might be effective.

Better, a buck converter designed for constant current.

I thought if before attaching the battery, I limited the output from the main battery BMS to 20 A that would be sufficient.

Dc-dc charger...victron Orion or any other brand that you can control the charge profile and C rate with

My main power station build has a DC-DC charger incorporated into it. If I want to, I could install a changeover switch that allows the DC to DC charger to output to a separate Anderson plug In charge the ammo case while driving. I would also have to change the C rate for the smaller battery.

I hope I'm not misunderstanding your intent but here's what I currently do.....
I bought a "Battery Tender" brand trickle charger and 'terminal harness' for the motorcycle battery on

I need to charge whilst out in the field and although I have an inverter that seems like a inefficient way to charge. Thank you for your reply however.

 

[robbob2112]

If your bms can reliably limit current to the C rate of the small battery that would work but it wouldn't be the correct profile. The dc-dc charger should actually be programed for a C rate and the profiles.

Either works, the dc-dc is a cleaner way

 

[Hedges]

BMS would only act as a circuit breaker, trip if the current is exceeded.

Closed-loop with inverter, it can ask for a particular charge level. Not sure it can ask for a maximum draw.

And when I said buck converter, that only works if source is higher than destination. DC/DC as suggested could be the right solution.
Small inverter and charger is OK. If you get 70% efficiency, good enough in this application. And both pieces are useful for other purposes.

When lithium cells are paralleled, that isn't considered to balance them. Parallel then charge above the upper knee parallels.
If your little battery is low and big battery is full, there will be voltage difference to transfer charge.
If big battery is 70% or 50%, it can't charge small battery except from below the lower knee.