Should i top balance now or wait?

Should I top balance my 16s battery now or wait until it's time to put into production in about 2-3 weeks? I still have to finish the power room, racking, inverter wiring. Which wouldn't take much but I have other projects going on and working and wife and kids and the whole lot.

My cells are all sitting at 3.27v. I just finished crimping my bms rings, and i'm a bit itching to be able to connect to the bms and check things out.

Top balancing takes a LOT of time, I’d start now, so they will be ready when you are.

At 3.27 it won't take long to top balance at all ... might as well wait .. I'm assuming he's not doing one at a time ...

You can top balance any time you like. If you do balance them now (you don't need to as the BMS will do it for you although slower) you might want to drain the battery to 80% afterwards as you don't want to keep it at a high state of charge for weeks.

3 responses and 3 different answers. Sweet.

They're 280ah batteries and i have no idea what SOC they are. I was thinking of taking them to 3.45 or 3.55 and then battery assembly. Something I can do after hours, where daylight is more building and going to home depot.

And yes, i'll be doing all at once.

If the BMS has an active balancer... consider skipping the top balance and just assemble and place in service.
May need to limit top charging voltage until things level out. I expect the cells will be very close right out of the box.

fatjay said:

They're 280ah batteries and i have no idea what SOC they are. I was thinking of taking them to 3.45 or 3.55 and then battery assembly. Something I can do after hours, where daylight is more building and going to home depot.

And yes, i'll be doing all at once.

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It sounds like you will top balance, in that case top balance to 3.65V, at that point they will all be very close together. Just remember to drain them down to to 80% after, they will store like that for a few months easily.

Enjoy your build Fatjay.

Bongbong said:

It sounds like you will top balance, in that case top balance to 3.65V, at that point they will all be very close together. Just remember to drain them down to to 80% after, they will store like that for a few months easily.

Enjoy your build Fatjay.

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What's the best way to drain them to 80% given their capacity?

There is no "best way", but you could assemble them into their final configuration add the inverter and use it to power your tools, a heater or whatever.

If the batteries are all the same voltage and assuming I trust the vendor and don't feel the need to check the capacity, I wouldn't even top balance them and I would just let the BMS take care of it over time. It's just too much work and there is a possibility to damage the soft metal terminals.

I was hoping there was some discharge tool that could be set to measure watts and automatically shut off at some point. I'm surprised there isn't already.

They sell electronic load testers. $50-to big money depending on what you want.

Only way to know where 100% full is on all cells is to fill them until they draw zero amps at above 3.4V only way to know where 50% is, is to drain them through a coulomb counting shunt meter.

If the cells are all new, and all same date, assembling into a pack with a bms you can monitor, you could go for it when you are ready, and just see if the bms has any runners.

Personally, i top balance everything i get now, because i have run into issues and digging everything out and balancing after the build is a pain.

Supervstech said:

Personally, i top balance everything i get now, because i have run into issues and digging everything out and balancing after the build is a pain.

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Yeah - this. Unless you have the JK BMS with the 2A active balancer (or add a stand-alone active balancer) it will never balance properly with what a typical bms with passive balancing can do.

I'd top balance. Most capacity issues here on the forum, assuming someone ordered decent quality cells, get traced to a lack of top balancing, or incorrect top balancing. A couple cells from both of my packs I built absorbed a lot more amperage than the others did, checked with a clamp-on meter on the balancing leads. Those cells would have eventually come into line, but would have caused me issues until then. I strongly prefer to negate issues proactively.

Top balancing doesn't take long, like a crock pot meal. Total elapsed time can be awhile, but most of the process is automatic and your direct involvement is minimal. Assemble in series as a normal battery with your BMS, charge until one cell hits high voltage disconnect. Reconfigure in parallel, charge at your preferred voltage until they stop absorbing amperage. Reconfigure back to series with your BMS, done.

If your cells all track together during that first charge, and start hitting the knees of the charge curve at the same time, you can decide then that it's close enough, and assemble into final form without top balancing - but with data to base your decision on. The first step is still the same.