Multiple Battery Banks

[Rabbit]

Hello!

I need to work out a way to efficiently charge one independent battery bank from another of the same voltage.

I have two 12v independent battery banks that are entirely off-grid. Both banks are lithium iron phosphate. One is fixed and immobile-- it consists of 4xLishen 280 ah, with 100 amp BMS's. The other is mounted on a custom-built cart complete with inverter, etc, for intermittent use in a cargo trailer. This mobile bank has 5 old-style (previous model) Renogy 100 ah batteries. When not in use in the trailer I store this cart at home and significantly discharge it perhaps once a week, but it's primary purpose is to service the cargo trailer. I wish to charge both of these banks from my 1200 watt 12-volt solar panel array. This was (IMO) improperly installed by someone else in that it's a single 1200 watt 12-volt string. Nor can I (for personal reasons) access this array to modify it in any way. (I have a Victron 150/100 controller that handles it well, however.) For reasons peculiar to the layout of my home, these two battery banks must routinely remain about a 30'/ten meter cable run apart. (This cannot be economically remedied.) Since I can't easily either unify the banks (due to the structure's physical layout, cable costs and routing difficulties for such large cables) or divide up the panels in such a way as to allow me to use more than one controller, what I'm trying to do is work out a means to use a DC/DC charger to charge the Renogy's from the Lishens, which have easiest access to the panel array. I've contacted both Renogy and a Victron dealer, the companies that make the best known DC/DC chargers, and both tell me that their products can't be used in any environment other than a vehicle. My next fallback would be to use a Victron 100/50 MPPT controller (that I already have on hand) to charge the cart, connected directly to the Lishen batteries. However, I'm reluctant to try this without some kind of confirmation that it's a safe and viable solution, and am concerned that I'd be attempting under some circumstances to "charge uphill". Another option I've considered is to buy a 12 volt to 24 volt converter, and then connect the MPPT controller to that. However, all of these I've seen are meant for golf carts. This isn't the sort of semi-continuous duty I have in mind. I sleep in the same building with this setup, and need _good_ gear to feel safe.

Any thoughts?

As an aside, even though it's so inefficient it hurts my eyes to look at it, as a temporary and experimental expedient I've been charging the cart using a Victron 12 volt charger plugged into an inverter connected to the Lishens. This has at least proven to my complete satisfaction that a 20-30 amp charger is all I need to supply my Renogy-cart under my own specific usage-case, and that my panels (even with this horrendous wastage) can supply all the power I personally need from both setups. So, all I have left is to find a solution to this bank-to-bank charging problem.

Thanks for all help!

 

[Diysolar123]

I find this an interesting comment:
"both tell me that their products can't be used in any environment other than a vehicle"
are you sure that they did not state/actually mean in a "protected environment"?

devices have no idea they are "in a vehicle"...its electrons and they do what they do ;-)
It would be important to meet the environmental restrictions on any device (humidity/temperature) wet exposure, etc.

 

[Rabbit]

They told me they absolutely, positively had to be wired to a vehicle's power-generating electrical system. I outlined my requirements as above, and they said "no".

 

[Diysolar123]

ok, but as an fyi... electrons have absolutely no "where I can from" special designators on them.
sometimes you may hear an answer from a company that is driven by the marketing/legal team...not saying that is the case here, its just, well, "odd"...

when I am working on solving a problem I break down the functionality I need and focus on the requirements that drives. If a device meets those needs then I use it, regardless if the marketing team thinks its a good idea or not hehe

when I need to simply charge a battery pack from a variable source, I use something like a isdt Q8 (smart DC-DC converter) and all is well.
If you can deal with the limited charge current such a simple device may be good enough.

it is important to understand the difference between buck/boost/buck-boost converters. In general the safest is a buck-boost DC-DC converter that is programmable. This type of device is one you never have to worry about a reverse current flow damaging your charger.

 

[Rabbit]

I found the vehicle requirement for the DC-DC chargers rather odd myself, as my desired setup is actually simpler and less demanding than that of a vehicle. But I'm not sufficiently skilled to confidently ignore manufacturer/dealer advice.

Thank you most kindly! Will the isdt Q8 hold up over multiyear near-continuous service, do you think?

 

[rickst29]

Rabbit, their "requirement" may be associated with an "ignition is on" sense signal failing to pass it's test (i.e., that "under the hood" Voltage confirms the engine to be running, and also high enough to allow for charging).

If that is a separate wire, you could probably run Lishen battery power into a tiny variable 12v Boost converter, dial up the output voltage to about 13.5V, and then run the DC-to-DC. But if it's an internal measurement against the source Voltage on the power leads, then your Boost Converter needs to handle very high power - and most of those are not variable.
- - -
Your 'alternate scheme #1' of using a 120v Inverter with an LFP "plug in" power converter is not awful, in terms of lost efficiency - you lose about 15-20% on each conversion, (total power loss 30-40%), but you save a bit of power by having the long wires at 120 VAC (low current) instead of 14 VDC (DC-to-DC charging at high current).
- - -
Your 'alternative scheme 2' (MPPT by itself) doesn't work without adding a Boost Converter at the beginning. An MMPT controller can only reduce the 'input' voltage to make more current, it can never increase the 'input voltage. The DC->DC charger combine a boost converter with a Solar Controller in one box. But, depending on the maximum watts you really need to keep the Renogy cart well-fed, you might of the option of boosting at the Lifshen bank, running higher voltage DC wires to the 'spare MPPT' charger, and using that charger at the Renogy Cart.

If you can get by with around 30 Amps, then all you need to add is this: https://www.ebay.com/itm/224532736008 that's a "Step Up Converter, 12v to 24V at up to 40A (960 Watts)

You would do the step-up at the Lishen, run 24v through the long wires, and use your "spare MPPT" at the Renogy end. The Victron 100/50 will want to use a maximum of about 50A * 14.4V / 93% efficiency = about 775 watts, and that's well below the limits of the step-up converter. Just be aware that the 40A wires will need to be AWG-8, or even larger (to reduce power loss). The Lishen connections, running up to almost 80A, would need to be AWG-4 or larger, but only for a short distance.

 

[Rabbit]

Thank you most kindly, Rick.

 

[Bubba1]

Perhaps a marine type battery selector switch. Turn off the solar panels to scc. Select the battery you want to charge and turn panels on.

 

[Rabbit]

Not a bad idea either. Thanks!