Inverter to inverter to charge batteries?

[Jim Fowler]

I have a small motorhome that I currently have a 280AH 24 volt lifepo4 batteries connected to a Victron MultiPlus 3000. I have another 16 cells I was going to add to the system, but it's turning into a bigger effort than anticipated due to limited space. I have a 20ft trailer I tow behind the motorhome quite often to haul our motorcycles and 3kw of solar panels mounted on the roof with a cable to connect to the motorhome to charge the batteries. I was thinking it would be easier to actually install the 16 cells in the trailer with another inverter, and then just plug the motorhome into that inverter to charge the batteries and provide extra capacity. If I were to get another Victron for the trailer, any idea how much I would lose in efficiency going from solar to batteries to inverter to motorhome inverter/Charger to run the motorhome and charge the batteries?

[Richard Bewza]

I have the same 7000wh battery pack and Victron MultiPlus in a fifth wheel.

Your idea is brilliant.

If you set the motorhome MultiPlus maximum input to 15 amps you should have a nice gentle transfer 

of your 14000wh trailer power to your motorhome loads and battery. Motorhome will remain mostly full except when motorhome needs PowerAssist because of a larger than 15 amp load. You will have 15x120=1800 watts plus 3000 watts for a 4800 watt load at times. You could power 2 air conditioners for quite some hours.

Rick 

[Dacian Todea]

Jim,

Victron inverter/chargers are fairly efficient so I will say loss should be less than 15%.  

But what is your larger energy user ? As the 280Ah battery at 24V seems already fairly significant and so is the 3000W solar array (actually that solar array if is not split in two seems a bit excessive for 280Ah battery as array should be able to output close to 100A in some conditions).

[Jim Fowler]

Dacian,

I live in the Seattle, Washington area where we get a lot of clouds and rain, especially in the winter so I oversized my panels to account for that.  It is also sized to allow for full solar power to my heaters/air-conditioners during the day while also charging the pack so they can continue to run through the night.  As an example, this winter I have been averaging only about 4-5kwh/day production from my panels.  I have them setup in a 2s5p configuration.  My motorhome is used as a home office/mobile office.  I am a consultant that used to travel a large portion of the time before COVID to my client's sites and so I use my motorhome instead of getting hotels all the time.  I'd rather my clients pay for a nice motorhome I and my family can use on occasion rather than making Marriott more rich.  So since I work out of the unit every day, even when parked at home, I keep it heated and cooled at all times.  This means that my biggest energy uses in the winter are electric heaters at almost 2000 watts worth, and in the summer it's air-conditioning at a similar draw.  The 280ah pack does not quite store enough to last through the night when I am sleeping in the motorhome while traveling in the winter as the sun goes down so early so have to heat off the batteries for more than 12 hours out of the day.  This is the reason for adding the additional capacity.

I am totally open to ideas for a better configuration as I am pretty new to the whole solar and lithium thing.  Already stripped some terminals on my batteries that I am trying to find the best way to fix too. :(

[Will OBrien]

If you must run a pair of separate packs, you could build a contactor pack to switch between them.

I just bought a six pack of 350a gigavac contactors for $150 that run on 12 or 24v on ebay. (They use them for forklift controls)

You could easily add a microcontroller to auto switch or just toggle it.

Unfortunately parallel SMBS0 packs isn't a supported option. (But it'd be nice Dacian!)

[Oberon Robinson]

Jim, have you considered installing a mini-split for your heating and air conditioning needs?  A heat pump will give you at least 2-3x the efficiency of your electric heaters.

If your battery terminals are female, I'd suggest that you drill & tap them oversize, and put in a thread insert.  Just make sure you have a depth stop when drilling, because if you go too far, things might get a bit too exciting.

[Dacian Todea]

Will,

Unfortunately parallel SMBS0 packs isn't a supported option. (But it'd be nice Dacian!)

It is not that SBMS0 is not supporting that. Lithium batteries packs can not be paralleled in any conditions only cells can be paralleled. If you want to parallel Lithium batteries it needs to be done trough something like an inverter or DC-DC converter.

For example Tesla Power Wall has a 12s battery inside but also an inverter so you can parallel multiple Power walls but you are not paralleling the batteries you parallel the output of the inverters.