Will a third lithium battery cause me any balance issue?

[Carpmart]

I’ve recently purchased a Fogster Drift 105A lithium for use with my trolling electric outboard when fishing.

For context (important to this question) I have two KS 204Ah batteries as the power store (habitation) for the Motorhome.

As I’m sat by a lake in France fishing at the moment, I was just musing to myself, how am I going to charge the new battery? Then I suddenly thought, 90% of the time, why not hook this Fogster battery up to the other two, giving me another chunk of battery capacity. It should also then be charged when I need it!

I then thought, what issues does it cause me having three batteries, one virtually half the capacity of the other two linked in parallel?

I’m assuming the BMS of the smaller battery will just stop any charging power supply from over charging and allow the other two to fully charge, but will the load be equalised when using the batteries in term of current draw? So will a modest 60A load from a travel kettle be draw from each battery equally? Does each contributes 20A to the current draw?

What happens when I’ve used 315A? If I’ve taken current equally form all the batteries, the Fogster will be fully depleted, yet I will drive have c. 200A (100A each) available from the two KS packs… is this how it works?

I’m a little confused.com

Edit: PS, I’m off grid 99% of the time so using solar only. I do have a multi channel Victron mains charger which I can use if on a hook up ever!

PPS, the battery compartment has space for the extra battery to be sited adjacent to the two existing KS’s ones, so only some short cables to hook up to existing batteries.

 

[bigtwin]

Any charge/discharge current will be distributed in proportion to each battery’s capacity; the current flow is determined by the internal resistance of each battery.

Ian

 

[jongood]

I’ve recently purchased a Fogster Drift 105A lithium for use with my trolling electric outboard when fishing.

For context (important to this question) I have two KS 204Ah batteries as the power store (habitation) for the Motorhome.

As I’m sat by a lake in France fishing at the moment, I was just musing to myself, how am I going to charge the new battery? Then I suddenly thought, 90% of the time, why not hook this Fogster battery up to the other two, giving me another chunk of battery capacity. It should also then be charged when I need it!

I then thought, what issues does it cause me having three batteries, one virtually half the capacity of the other two linked in parallel?

I’m assuming the BMS of the smaller battery will just stop any charging power supply from over charging and allow the other two to fully charge, but will the load be equalised when using the batteries in term of current draw? So will a modest 60A load from a travel kettle be draw from each battery equally? Does each contributes 20A to the current draw?

What happens when I’ve used 315A? If I’ve taken current equally form all the batteries, the Fogster will be fully depleted, yet I will drive have c. 200A (100A each) available from the two KS packs… is this how it works?

I’m a little confused.com

Edit: PS, I’m off grid 99% of the time so using solar only. I do have a multi channel Victron mains charger which I can use if on a hook up ever!

PPS, the battery compartment has space for the extra battery to be sited adjacent to the two existing KS’s ones, so only some short cables to hook up to existing batteries.

could you not get one of the self-contained stand alone panels and clip it to the battery from time to time

 

[Carpmart]

Any charge/discharge current will be distributed in proportion to each battery’s capacity; the current flow is determined by the internal resistance of each battery.

Ian

Thanks Ian

Much clearer… so if the IR is less on the smaller battery than the two bigger ones, it could end up doing (proportionally) more of the work, is that correct?

 

[Jimbohorlicks]

All this thinking, Harvey, is because your not catching any fish.
If you link up your 2 x204Ah batteries to the 1x 105 battery, you should theoretically have 509 Ah of power.
.it's very unlikely you are going to use 315ah at any one time anyway ,unless that is you catch a big enough to eat fish for your airfryer.
If you do,the bms on the 105ah battery should cut in and prevent the battery totally discharging..and the balance of the amps would come from the 2 x204ah.
Another way to chargecup the 3rd battery would be to fit a battery master, one that's configured for Lithium, the type that Hoovie recommends.
Best of luck with the fishing.

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[Carpmart]

could you not get one of the self-contained stand alone panels and clip it to the battery from time to time

As in solar panel?

Yes I could, but if we are having some inclement weather, some additional main habitation capacity would be nice…

 

[Jimbohorlicks]

As in solar panel?

Yes I could, but if we are having some inclement weather, so additional main habitation capacity would be nice…

But you would still be relying on your roof solar for charging up.

 

[bigtwin]

Thanks Ian

Much clearer… so if the IR is less on the smaller battery than the two bigger ones, it could end up doing (proportionally) more of the work, is that correct?

If it was, it would, but it isn’t so it wont.

Smaller batteries in a bank of larger batteries will provide/receive current in proportion to their size. In other words, it would provide less current to any load. In other, other words, each battery will deplete by the same percentage such that they are, at all times, at an equal (%) state of charge/discharge.

Ian

 

[Carpmart]

If it was, it would, but it isn’t so it wont.

Smaller batteries in a bank of larger batteries will provide/receive current in proportion to their size. In other words, it would provide less current to any load. In other, other words, each battery will deplete by the same percentage such that they are, at all times, at an equal (%) state of charge/discharge.

Ian

Thanks Ian, really helps my understanding….

 

[Carpmart]

But you would still be relying on your roof solar for charging up.

I will indeed Jim, but I can run for a few days without solar; this would give me potentially another day off grid before driving (B2B), Gennie or hook up is needed if there is zero solar!

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[Raul]

Thanks Ian

Much clearer… so if the IR is less on the smaller battery than the two bigger ones, it could end up doing (proportionally) more of the work, is that correct?

The bigger the cell, the lower the resistance. A 18650 2-2.5ah cell has a tipical resistance of about 1.5 miliohms, a 280ah EVE cell has 0.16-0.18 miliohms. Huge difference.
As bigtwin said already, it will contribute in equal percentage of its capacity. This assuming the paths are equal to, as in cabling, connection and fuse.
The only thing I would look out for, it’s charge settings. I would run 14.2-13.5v conservative. Reason is two different makes of bms and bms settings. You want to avoid triggering any bms protections.

 

[Wellerfan]

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