Going without a BMS for a month. Is this a bad idea?

[Freep]

I've been putting together a 271ah (4s 12v) battery for the last few days and running into issues with my BMS limiting current when I put a load on it. I'm thinking about just dropping the BMS altogether. I just ordered an overkill BMS that is supposed to arrive on Thursday but we're going to leave in our camper on Friday so I'm cutting it pretty close.

I have a Victron smart solar MPPT controller and a Victron DC-DC Orion-TR charger and they should be enough protection from overcharging. Because we will be living in a truck camper for a month I will be able to monitor the battery voltage and SOC closely with this battery monitor which will allow me to set alarms for over/under voltage conditions. Plus I have this little balancer to rebalance the cells if needed and it's already wired up and working.

So should I just plan on not having a dedicated BMS and keep a close eye on it and trust my Victron chargers to not overcharge? Or should I punt and put those heavy AGMs back in?

 

[gnubie]

If you can rely on your cells having closely matched impedance you could get away with it, but if they aren't you will push some cells to higher voltages during charging, potentially overcharging them. If you are going to do this you'd want to test your cells first to work out their impedance to see if its even practical, and then if so keep a close eye on the individual cell voltage during charging.

Any external charger will only see and react to the overall battery voltage. They won't know if one cell is frying.

 

[John Frum]

If you have to ask, probably.

 

[DiyNuke]

Absolute worst case scenario would be that you wreck the cells. But i doubt if that will happen. But how about putting the bms parralel on the battery so its does balance but just doesnt have the overcharge/overcurrent protection.
Mppt will take care of the overcharge and an fuse for the overcurrent.

 

[Freep]

Absolute worst case scenario would be that you wreck the cells. But i doubt if that will happen. But how about putting the bms parralel on the battery so its does balance but just doesnt have the overcharge/overcurrent protection.
Mppt will take care of the overcharge and an fuse for the overcurrent.

I can't trust this BMS at all. I just went to check on the system and the solar charger was charging but the "battery" was at 3.2 volts. Nothing was on in the camper. I bypassed the BMS and checked the battery and cell voltage and the battery was at 13.7 and the cells were at 3.425

The BMS is just junk. Or at least it's junk the way it's setup and I have not been able to connect to it to see what the parameters are or change them.

 

[John Frum]

I can't trust this BMS at all. I just went to check on the system and the solar charger was charging but the "battery" was at 3.2 volts. Nothing was on in the camper. I bypassed the BMS and checked the battery and cell voltage and the battery was at 13.7 and the cells were at 3.425

The BMS is just junk. Or at least it's junk the way it's setup and I have not been able to connect to it to see what the parameters are or change them.

Maybe the bms has tripped or maybe the balence leads are out of sequence.

 

[Freep]

Maybe the bms has tripped or maybe the balence leads are out of sequence.

I checked those leads multiple times because I did put them in the wrong order the first time I wired it up and got really paranoid about it.

Black -- Battery Negative
Red1 -- Cell one positive (same cell that battery negative is on)
Red2 -- Cell two positive
Red3 -- Cell three positive
Red4 -- Cell four positive

I've also installed the leads for the little balancer and that works fine.

 

[GMB]

You could bottom balance the cells and charge them up and when a cell reaches 3.5 or 3.6 volts, note the battery pack voltage and set the Victron to limit the charge to that point using the Victron app. Monitor the individual cells daily with your BattGo until you return to install the new BMS.

 

[John Frum]

I checked those leads multiple times because I did put them in the wrong order the first time I wired it up and got really paranoid about it.

Black -- Battery Negative
Red1 -- Cell one positive (same cell that battery negative is on)
Red2 -- Cell two positive
Red3 -- Cell three positive
Red4 -- Cell four positive

I've also installed the leads for the little balancer and that works fine.

OK one thing ruled out.
Perhaps it's tripped.

 

[John Frum]

I would walk the connector end of the leads with a dvom. Make sure the voltage increments as expected.

 

[JoeHam]

If I understand your problem it’s only during discharge.

If that’s correct then leave the BMS for charging and go directly to your terminals for your load. Just check with a VOM to avoid over discharge or perhaps your load has a low voltage alarm.

 

[Freep]

OK, I'm an idiot. I checked and rechecked to see that the leads were in sequence, but I didn't check the harness itself. After getting a new BMS with Bluetooth and wiring the sucker up I saw this.

I freaked out for a few seconds then hit the cells with my meter and they were fine. Then it dawned on me that I used the same leads I had from the other BMS because I didn't feel like crimping. It turns out that one of the leads has an intermittent connection.

So I used the leads from my little balancing device and everything looks fine now.

 

[rmcrowe]

I'm thinking about going without my bms for probably 6 weeks. In my case I plan to use an sbms0 but stock won't be in until late August. I imagine it might drag out further than that... same situation - we have some travel plans in the RV...

I have just finished testing 4 280ah cells from Shenzhen basen on Alibaba, and they seem spot on. ... Ir was .11 for all but one which was .10. This was at about 70% soc so not perfect measurement but i think close enough. I did a capacity test on each cell and they were all right at 275 (I only took them down to 2.9v, so they probably had the other 5ah in them).

I then balanced them to 3.2v in parallel and then charged them in series up to 13.3v. The cells all measured within .01 volt of each other after that so I have confidence I should be able to run without a bms for a short while (6 weeks?). I will top balance again before final configuration.

I have a victron smartsolar mppt and multiplus and I was thinking to set them pretty conservatively, like discharge to 12v and charge to 13.3v... that won't give me all of the battery but it will be plenty for my needs.

Any thoughts on this?

 

[JoeHam]

If you stay well within the flat part of the discharge curve (as I believe you plan to do) then you should be fine.

If you haven’t seen the curve let me know and I will find one to post.

Basically you want to avoid the upper and lower knee area.

 

[Fred S]

I would check each cell after a full charge with the Victron the first few times, to make sure they're not wandering too much. After that, at those settings there is very little risk.

 

[rmcrowe]

Thanks, yes I plan to stay in the flat part of the discharge curve while running without BMS, and I will indeed test the cells with a few charge/discharge cycles just to double check. Appreciate the feedback.

 

[Fred S]

Have a fun trip!