Parallel packs/overkill BMS pros/cons

[Freqz]

I have 48 202Ah Lishen cells on a slow boat from China for my new build. During the wait, I’m considering the rest of the system.

At a glance:

• Mobile 48VDC/240VAC split phase system in a RV with 240VAC/120VAC/12VDC loads
• Blue Box electronics with Victron inverters (undetermined exact VA rating— probably10k combined) and as much solar as possible crammed on the roof (expecting ~3000W on my 33’ fifth wheel)
• System reliability/redundancy/fast field troubleshooting top priority (when I’m deployed I work multiple 100 hour weeks in terrible environments and just don’t have time to tinker until I’m on days off)
  

Looking at bettery arrangements, I’m considering 3p16s vs 16s3p. 16s3p gives me options if something goes bad with a cell while I’m working. That is, I can disable one pack and run off two and tinker/repair later. If I do that, the BMS consideration is a Victron smart shunt handling SOC/charging with overkill solar acting as a safety vs. a Orion Jr. handling it all in a 3p16s configuration (keeping dollars roughly equal). Ideally, the Victron should keep everything out of the knees of the combined pack (I like more battery run conservatively vs less battery run to the limits since I’m not super weight sensitive) and I don’t have to think about it again for the next several years but if something dumb happens, the overkill shuts off the packs until I can disconnect the offender and go about business with two until I get the opportunity to fix it.

Obviously, one pack is easier and the Orion Jr. is great but I gain added troubleshooting/redundancy options with 3x Overkill protected packs.

Thoughts?

Thanks!

 

[snoobler]

16s3p gives me options if something goes bad with a cell while I’m working. That is, I can disable one pack and run off two and tinker/repair later. If I do that, the BMS consideration is a Victron smart shunt handling SOC/charging with overkill solar acting as a safety vs. a Orion Jr. handling it all in a 3p16s configuration (keeping dollars roughly equal). Ideally, the Victron should keep everything out of the knees of the combined pack (I like more battery run conservatively vs less battery run to the limits since I’m not super weight sensitive) and I don’t have to think about it again for the next several years but if something dumb happens, the overkill shuts off the packs until I can disconnect the offender and go about business with two until I get the opportunity to fix it.

Obviously, one pack is easier and the Orion Jr. is great but I gain added troubleshooting/redundancy options with 3x Overkill protected packs.

You've pretty much covered it.

I would add that 3 batteries in parallel are at the same voltage. If one has a failing cell, and the pack voltage drops, the others will discharge into it, so the "overkill shuts off the packs" isn't particularly fast or reliable until a single cell drops permanently below the LVD and the other batteries are freed from the chains of the misbehaving battery.

Additionally, a 3P16S has a similar problem. ONE bad cell may take out it's cellmates. Your whole battery may be down for an extended period as you find which of the 3 cells is bad.

 

[Topper]

As for my agm and limited lifepo4 experience, it is a good option using 3 independent strings.
My own 2 8s strings on 2 Overkill BMS's, shows some distinct differences between the 2. Charge and discharge rates vary 7%. And like my run to the bottom today, 1 bms low cell disconnected while the 2nd still had some life left.
Personally i prefer multiple BMS's if they are properly sized for the application and intended loads . It just makes it easier to manage problems if they should arise while still keeping the power on.

 

[jwelter99]

If your use case is correct I would buy 2 48V 5kw storage system, put them in parallel, and call it done. Get your accountant to claim it as a business expense along with the rest of the RV.

 

[snoobler]

If your use case is correct I would buy 2 48V 5kw storage system, put them in parallel, and call it done. Get your accountant to claim it as a business expense along with the rest of the RV.

Why are you recommending he only install less than 1/3 of his intended battery capacity?

 

[Freqz]

You've pretty much covered it.

I would add that 3 batteries in parallel are at the same voltage. If one has a failing cell, and the pack voltage drops, the others will discharge into it, so the "overkill shuts off the packs" isn't particularly fast or reliable until a single cell drops permanently below the LVD and the other batteries are freed from the chains of the misbehaving battery.

Additionally, a 3P16S has a similar problem. ONE bad cell may take out it's cellmates. Your whole battery may be down for an extended period as you find which of the 3 cells is bad.

Yeah, but it seems easier to my (admittedly compromised engineer brain) to identify one bad pack of three vs one cell of 48. And that gives me options until I get back to days off and have more time to tinker. Shifting problems to days off seems to be my biggest hobby ?

 

[Freqz]

If your use case is correct I would buy 2 48V 5kw storage system, put them in parallel, and call it done. Get your accountant to claim it as a business expense along with the rest of the RV.

That’s 1/3 of the capacity for twice the dollars (at least). I don’t mind putting in hours up front— I work ~26 hard weeks a year with 26 weeks off so I have time to get it set up right (and I *like* to tinker when I have the time)

 

[Freqz]

As for my agm and limited lifepo4 experience, it is a good option using 3 independent strings.
My own 2 8s strings on 2 Overkill BMS's, shows some distinct differences between the 2. Charge and discharge rates vary 7%. And like my run to the bottom today, 1 bms low cell disconnected while the 2nd still had some life left.
Personally i prefer multiple BMS's if they are properly sized for the application and intended loads . It just makes it easier to manage problems if they should arise while still keeping the power on.

Thanks. I’d run these in FET bypass mode (or whatever you want to call it)— limp mode of two packs will likely draw more than I’m comfortable sending through one overkill BMS. I’d like a FETless version of overkill: it’d likely save a few bucks and suit the kind of big cell stuff so many of us are doing a little better.

 

[snoobler]

Yeah, but it seems easier to my (admittedly compromised engineer brain) to identify one bad pack of three vs one cell of 48. And that gives me options until I get back to days off and have more time to tinker. Shifting problems to days off seems to be my biggest hobby ?

In reading my reply, I didn't make it clear.

3 packs, 3 BMS = best.

 

[Ampster]

If I do that, the BMS consideration is a Victron smart shunt handling SOC/charging with overkill solar acting as a safety vs. a Orion Jr. handling it all in a 3p16s configuration (keeping dollars roughly equal).

Check out the install @cinergi with Victron and Orion Jr. The integration of the BMS with the Victron appealed to me. I have been using an Orion BMS for six years and wish I could have that kind of integration with my Outback Skybox.

 

[Freqz]

In reading my reply, I didn't make it clear.

3 packs, 3 BMS = best.

I’m like 99% there. My current setup up is 4 parallel 24V packs, 0 BMS but that’s a different animal entirely. I considered Batrium, but that would double my BMS cost and the WAF is near zero on this as it is, lol.

 

[Freqz]

Check out the install @cinergi with Victron and Orion Jr. The integration of the BMS with the Victron appealed to me. I have been using an Orion BMS for six years and wish I could have that kind of integration with my Outback Skybox.

Actually, his RV was built by the same outfit I’m using (and a FB post of his pointed me here). Our loads are similar but his use case is a tad different from mine— his spare time seems to be more evenly distributed and gets to chase better weather. I saw >120° F swing this year in my RV.

I’m still suffering PTSD from the historic cold snap in TX a couple of months ago which has me rethinking A LOT of how my system is built and it’s capabilities.

 

[jwelter99]

Why are you recommending he only install less than 1/3 of his intended battery capacity?

3kw of solar and the ability to add another pack when/if needed to find the right balance.

But my advice was off as OP points out he likes to tinker and does have substantial free time to get things up and working before the work-slog starts. So then the DIY likely makes sense.

 

[cinergi]

Actually, his RV was built by the same outfit I’m using (and a FB post of his pointed me here). Our loads are similar but his use case is a tad different from mine— his spare time seems to be more evenly distributed and gets to chase better weather. I saw >120° F swing this year in my RV.

I’m still suffering PTSD from the historic cold snap in TX a couple of months ago which has me rethinking A LOT of how my system is built and it’s capabilities.

pretty accurate tho right now I am in Connecticut and dealing with snow and freezing weather, blowing thru my 40 pound propane tanks. System is still performing wonderfully. I ran my generator for 2 hours and that’s it - haven’t plugged in for 2 weeks.

 

[Freqz]

pretty accurate tho right now I am in Connecticut and dealing with snow and freezing weather, blowing thru my 40 pound propane tanks. System is still performing wonderfully. I ran my generator for 2 hours and that’s it - haven’t plugged in for 2 weeks.

I’m probably going with the static 100lb tank and *maybe* a 40 lb bottle— but I’m at a 33’ floor so fitting the mini split in that basement is a new world for NH. I’ll likely be their shortest build yet with one in place. I really wanted a diesel aqua hot but had to draw the line somewhere in my build.

 

[cinergi]

I’m probably going with the static 100lb tank and *maybe* a 40 lb bottle— but I’m at a 33’ floor so fitting the mini split in that basement is a new world for NH. I’ll likely be their shortest build yet with one in place. I really wanted a diesel aqua hot but had to draw the line somewhere in my build.

I wasn't supposed to end up with a Truma/propane but long story short, I did; I can tell you I'd rather have a traditional 12 gallon propane. Diesel's another story - I have a diesel Oasis on the Newmar and that's better .. but if the incoming water is cold, it can't keep up with a shower (it will be pretty warm, but not hot; depends on how you like your shower). The Oasis is pretty noisy, too . I dunno about the aqua hot. the propane stuff (both the heat and hot water) is nice 'n quiet. Plus I only have to worry about 1 source of fuel and I don't have to worry about things gumming up if it sits too long etc etc. The same propane will be for generator and grilling. I thought 2 40 pound bottles would do better ... but I forgot that 40 pounds = 9 gallons or so ... which isn't that much.

Hm.. mini split in a 33... are you still going to have 2 9k cassettes?

 

[HRTKD]

@Freqz, are you sure you can fit that much solar on the roof? RV roofs are notorious for poor space efficiency. Maybe you're a better Tetris master than I am.

My vote is for the three independent batteries. In your situation, redundancy is good. Three BMS = 300 amp of capacity.

 

[Freqz]

I wasn't supposed to end up with a Truma/propane but long story short, I did; I can tell you I'd rather have a traditional 12 gallon propane. Diesel's another story - I have a diesel Oasis on the Newmar and that's better .. but if the incoming water is cold, it can't keep up with a shower (it will be pretty warm, but not hot; depends on how you like your shower). The Oasis is pretty noisy, too . I dunno about the aqua hot. the propane stuff (both the heat and hot water) is nice 'n quiet. Plus I only have to worry about 1 source of fuel and I don't have to worry about things gumming up if it sits too long etc etc. The same propane will be for generator and grilling. I thought 2 40 pound bottles would do better ... but I forgot that 40 pounds = 9 gallons or so ... which isn't that much.

Hm.. mini split in a 33... are you still going to have 2 9k cassettes?

I’m still in draft...we’re pretty sure the ODU will occupy space for one bottle. If I have my way, I’ll have 18k/12k cassettes. My climates tend to very hot/dry or very cold (in which case I’ll have a external tank set for the winter). I’d give up a bit of cabinet space or have low static ducted with no “hat” and get another 800w of solar up top but we’re still working out the final details.

 

[Freqz]

@Freqz, are you sure you can fit that much solar on the roof? RV roofs are notorious for poor space efficiency. Maybe you're a better Tetris master than I am.

My vote is for the three independent batteries. In your situation, redundancy is good. Three BMS = 300 amp of capacity.

Yes...I’m specifically having it built with minimal roof protrusions and have some wiggle room in placement of components and whatnot. No TV antennas, no vent for a propane fridge that will never exist, no skylights, mini split ODU in the basement, etc.

 

[HRTKD]

Yes...I’m specifically having it built with minimal roof protrusions and have some wiggle room in placement of components and whatnot. No TV antennas, no vent for a propane fridge that will never exist, no skylights, mini split ODU in the basement, etc.

Oh, OK. I didn't realize this was a custom rig. Nevermind.