Monitor married with active balancer

[THUDGUN]

So, I'm trying something a bit different. Instead of putting a BMS on each battery pack, I took a voltage monitor and wired in an active balancer. This monitor is good up to a 24s pack, though I'm just using 13. It shows total voltage, the highest and lowest as well as the difference. The idea is, once a month, I'll go plug in the monitor to each pack, in turn, and if any need balancing, just leave it connected for about an hour, which seems to do the trick, so far. Regular maintenance. So far, this appears to be working just as expected, but the pack has never been significantly out of balance.

I don't trust BMS' and the decent, smart ones are too expensive for my build, at this time. Eventually, I'll probably add a smart BMS to each battery pack, eventually, but I figure this would work well till I could afford the upgrade.

So here's a 13s4p battery pack for my 48 volt power wall. I'm using a ribbon cable for all the cabling. makes wire management so much cleaner. Added the connectors for easy plug and play. These will also sever as the connections for whatever BMS I eventually add, so super convenient and flexible. I'll just have to connect the charge leads.

Second pic is the monitor with the active balancer on the bottom. You can make out a series of cans on it...


So, jut looking for a bit of feedback on whether this is a good idea, or what things I need to be weary of, going forward.

Thank you kindly.

 

[time2roll]

Has the balance been checked at maximum and minimum battery voltage?
Can't recommend going without protection at the cell level. Still may work fine as long as everything stays within the expected parameters.

 

[THUDGUN]

Has the balance been checked at maximum and minimum battery voltage?
Can't recommend going without protection at the cell level. Still may work fine as long as everything stays within the expected parameters.

Haven't gotten that far yet. Just wired up the monitor/balancer. Still have to wire up the charge controller and power wall. I don't have a smaller unit to charge and discharge for testing, so it'll happen on the full unit. Seems all the battery packs are pretty well balanced (from Battery Hookup). The pack I'm building from scratch (salvaged batteries) will require significant balancing, I'm sure, so that'll be a good test, though that pack won't be going into the wall. Wasn't happy with the capacity... Still, I'm learning a lot with that process.

As for cell-level protection, they all have cell level fuses, and I'm putting a 150A fuse on the positive terminal. I can't imagine a pack going out of balance fast enough that I won't catch it with regular maintenance, but the charge controller has thermal runaway protection, so if the battery does get hot, it'll cut out, so it's not like there is no protection without the BMS. Maybe I'll put a cheap BMS on them, but I really don't see the point, as I won't know if it's working or a bad unit till the packs go out of balance anyway... Probably just going to save up for decent smart BMS' and add them as soon as I'm able to.

 

[time2roll]

The cell protection relates to over voltage and under voltage. These situations are easily common at the top or bottom of the cell voltage range.
Everything can look great in the middle 80% to 90% and still have an issue at the ends. May not cause a fire or heat but rather accumulate a loss of capacity over time.
Be conservative with the capacity for best results.

 

[THUDGUN]

Right. I wasn't planning on charging or discharging to max, regardless. Rather have longevity over peak capacity. If I had the money for that, I'd just buy manufactured solutions instead of building my own. !)

 

[brb58]

Right. I wasn't planning on charging or discharging to max, regardless. Rather have longevity over peak capacity. If I had the money for that, I'd just buy manufactured solutions instead of building my own. !)

You need something to disconnect for under/over voltage. I have accidently left something on and completely discharged my lithium batteries so even if you don't mean to, it can still happen.

 

[THUDGUN]

You need something to disconnect for under/over voltage. I have accidently left something on and completely discharged my lithium batteries so even if you don't mean to, it can still happen.

The charge controller I got does this. Not on the individual packs, but overall. I won't be charging to full or discharging to 'dead', plus all the batteries are identical, so I doubt it will run away fast enough that it isn't caught in regular maintenance. If the batteries continually go out of balance, or if I have cutouts, heat, etc, than I'll revisit it. My intention is to eventually add the proper BMS to each pack, but at the moment, it's just an additional cost I can't afford, so diligence is my substitution. This is just a test, to see how it goes.

 

[brb58]

Your charge controller can limit the discharge? Using what? A relay?

What is going to be your upper voltage limit ?

 

[THUDGUN]

Your charge controller can limit the discharge? Using what? A relay?

What is going to be your upper voltage limit ?

It's an MPPT controller. I can set the maximum and minimum voltage as well as the rate of charge and discharge. The charge controller will switch from inverter to utility when the voltage is too low, and back to battery when voltage reaches the max set. It should not be possible to get higher than the max setting, if the controller works properly, which I will be testing, once I get it all assembled.

I don't know the actual max and min of the battery packs yet. Once I have those figures, I'll be determining what minimum and maximum I'll be setting. Probably at 90% of max. The controllers max limit is 58.8v and the lower limit is 39.6. I don't plan on getting close to those figures in the final setting, but again, need to know the health of the packs before I do set that.

 

[brb58]

Please show what MPPT controller can con control discharge. Did you mean your inverter?

In any case, that's a very poor way to do it. You could set a minimum voltage and one of the cells could be way lower than the others and the pack will still be above minimum but not that one cell and it will get damaged. You need something that monitors every cell for both high and low voltage.

 

[mikefitz]

My guess is that your cells are not LFP, and thus have a greater risk of thermal runaway, fire and disaster.
Having individual continual cell monitoring with a BMS would seem important for safety.

Mike

 

[time2roll]

Please show what MPPT controller can con control discharge. Did you mean your inverter?

In any case, that's a very poor way to do it. You could set a minimum voltage and one of the cells could be way lower than the others and the pack will still be above minimum but not that one cell and it will get damaged. You need something that monitors every cell for both high and low voltage.

Some have a load control but power is a bit limited.

 

[THUDGUN]

Please show what MPPT controller can con control discharge. Did you mean your inverter?

In any case, that's a very poor way to do it. You could set a minimum voltage and one of the cells could be way lower than the others and the pack will still be above minimum but not that one cell and it will get damaged. You need something that monitors every cell for both high and low voltage.

It's the Renogy 48V Inverter Charger. Yes, it has the inverter built in and I can set the rate of charge and discharge.
I understand that it is possible for the pack to become so out of balance, that some sells can be over-charged/discharged, but this is unlikely in the short term. With regular monitoring and maintenance, I believe there is little chance of that. The battery cells are identical (as identical as manufacturing allows). They're going to be used for failover, so won't be regularly discharged and I will be monitoring each pack regularly, so detecting out of balance packs will be brought back into balance. I understand this is not a perfect solution, but this is where I am at, at the moment.

 

[THUDGUN]

My guess is that your cells are not LFP, and thus have a greater risk of thermal runaway, fire and disaster.
Having individual continual cell monitoring with a BMS would seem important for safety.

Mike

They're LiIo, but there are cell-level fuses, so thermal runaway is unlikely.
I plan on eventually adding a BMS to each pack, when I can afford one I can trust, and can monitor remotely (smart BMS). They're just too expensive for me at the moment.

 

[THUDGUN]

I think I have a good list of all the pitfalls I need to keep on top of, for this experiment. Thank you all.