Paralleling batteries of different capacity?

[HaldorEE]

I have a van conversion project in process and live in Phoenix AZ. I am concerned about battery longevity during the hot summer months and would like to only install the the primary house battery when using the van for boon-docking.

I would like to be able to use the ventilation fan and LED lights in the van with the primary house battery removed. My thinking is to add a smaller, lower cost, battery to replace the primary battery so the SCC will continue to output power for the fan and lights. Assuming the small battery has the same chemistry and voltage as the large house battery and has its own BMS, is there any reason the two batteries couldn't be wired in parallel? I would be OK with replacing the small battery every so often, but would whimper a bit if the 3000-WH battery died prematurely.

 

[Bob142]

You could just use one battery at a time where each battery has an Anderson SB connector to allow for easy swapping. If you're talking about a 12V system it's probably best to go with the SB350 connectors.

 

[Pierre]

I have a van conversion project in process and live in Phoenix AZ. I am concerned about battery longevity during the hot summer months and would like to only install the the primary house battery when using the van for boon-docking.

I would like to be able to use the ventilation fan and LED lights in the van with the primary house battery removed. My thinking is to add a smaller, lower cost, battery to replace the primary battery so the SCC will continue to output power for the fan and lights. Assuming the small battery has the same chemistry and voltage as the large house battery and has its own BMS, is there any reason the two batteries couldn't be wired in parallel? I would be OK with replacing the small battery every so often, but would whimper a bit if the 3000-WH battery died prematurely.

What chemistry is your primary house battery ?

 

[HaldorEE]

I plan on using Anderson SB175 connectors for the primary house battery. It is a 25.9V, 120A lithium Ion battery pack. Made out of these LG automotive cells in a 7S configuration.

LG Chem N2.1 3.6V 120Ah cell module

We are listing these every business day. If you see sold out there will be more listed at the end of each business day. They are selling faster than we can test them at the moment. This is a rare opportunity to get these cells that have never been cycled. LG 120ah cell modules. There are 2 60ah...

batteryhookup.com

That is not a bad idea of using a second SB175 for a second smaller battery. I was just planning on mounting the second battery permanently in the system unless doing so would be a bad idea.

Here is the rest of the system:

• Victron Multiplus 24/2000-50A
• Victron Smartsolar MPPT 100/20A
• Victron Orion-TR Smart 12/24-15 DC-DC charger
• Plus a couple of Victron Orion 24/12-5A DC-DC converters to power point 12V loads

I really like the LG cells because they are very compact and have very high energy density. I am mounting them in a 40mm Ammo Can with passive cooling because of the "vent with fire" issue. Battery construction is still underway.

 

[HaldorEE]

Gnubie tested the SSC I am planning on using (Victron Smart Solar 100/20) and verified that it will "remember" the battery voltage and output the correct float voltage even without a battery connected.

That means I can just unplug the house battery any time I want and continue to use the solar panel to power the van's ventilation fan and LED lighting (as long as the sun is still shining). This is exactly what I had hoped to do in the first place.

Thanks for your time all.