Large Voltage Drop on BMS Output While Under 110a Load

[OTRwSolar]

I'm using a Daly Smart 4S 200a BMS with 4 3.2 200ah cells. With a 110a load the cells are producing 12.90v, however, the output of the BMS is only producing 12.15v.

That's a huge .75v drop. The 12.15v reading is being taken on the output of the BMS (same reading on both sides of the BMS black output cable---ruling out this cable as a problem). I've also checked the voltage on both sides of the BMS blue (input) battery (-) cable---both sides read 12.90v (while under >110a load)---ruling out a problem with that cable, too.

In any case, that's a huge voltage drop that's internal to the BMS itself. The BMS is not reporting any faults. It will support a 110a load for 30-60 minutes without an issue. Unfortunately, I can only discharge my cells to about 20-25% at 110a before the low voltage alarm on my inverter activates. If I use a >110a load, it alarms even earlier.

I know the mosfets will drop the voltage somewhat, but .75v seems a bit unreasonable. Is this the price we pay for a higher rated (>120a) BMS? I have a friend of mine with an off-the-shelf LifeBlue LifePo4 100ah battery that produces 100a at 12.65v.

 

[John Frum]

How thick is the cable from the battery positive all the way back to the battery negative?
How long is the round trip circuit?
Are there any hot spots?
Check all the connection points.

 

[OTRwSolar]

Thanks for the reply! One end of each input/output cable is connected directly to a large metal bracket that goes directly inside the BMS itself. I measured both of these points at the same time with the 110a load. Measured 12.90v on the metal bracket input (cell) side of the BMS and 12.15v on the metal bracket output side (with the 110a load). There may be a .01v voltage drop from the cells to the BMS. The .75v voltage drop is internal to the BMS. If there was a cabling or connection issue it would be reflected in a lower cell voltage, too. Nothing like that taking place. FWIW, I have a .15v voltage drop from the BMS output to the inverter input at 110a (using less than 10 ft. of 4/0 cable). If I were to connect the cells directly to the 110a load without the BMS I'm very confident I would read 12.90v at the cell terminal itself (with a 110a load).

Really curious what voltage drop others are measuring with Daly Smart 150/200/250 and other brands of BMS's with a 100+ amp load.

 

[John Frum]

There is no other explanation for the drop.

If you are qualified to say there is no other explanation then you don't need my help.

 

[OTRwSolar]

Sorry, poor choice of words. Please let me clarify. I do need help. However, my question is not really where the voltage is drop is occurring, but *why* the BMS is dropping .75v. If others have significantly less BMS voltage drop (inside the BMS, not via external circuitry or wiring) under heavy load, then I may have a defective unit. Or, it maybe it's normal for the Daly. I don't know. Thanks!

 

[John Frum]

Sorry, poor choice of words. Please let me clarify. I do need help. However, my question is not really where the voltage is drop is occurring, but *why* the BMS is dropping .75v. If others have significantly less BMS voltage drop (inside the BMS, not via external circuitry or wiring) under heavy load, then I may have a defective unit. Or, it maybe it's normal for the Daly. I don't know. Thanks!

What is the voltage differential between bms b- and p- ?
I mean put one probe on b- and the other on p- ?
With current flowing of course.

 

[Ampster]

Can you vary the loads to see if it is related?

 

[OTRwSolar]

What is the voltage differential between bms b- and p- ?
I mean put one probe on b- and the other on p- ?
With current flowing of course.

Thanks! Will make a point of checking that tomorrow.

 

[Ampster]

Thanks! Will make a point of checking that tomorrow.

If there is voltage drop there could be heat. Check for that also.

 

[time2roll]

82.5 watts of heat is going somewhere.

 

[OTRwSolar]

Can you vary the loads to see if it is related?

I don't have a variable load bank, but I do have a 1200w ceramic heater that ramps up current very slowly when you first turn it on. I noticed the BMS's voltage drop stays relatively low, around .1v - .2v up to 40a or so. Between 50-70a it ramps up very quickly to .6-.7v. and only rises slightly to .75v as the heater continues to ramp up from ~70a to 110a. Definitely far from a linear response. While all this is going on, the battery/cell voltage (VM connected directly to the terminals on the cells) dropped in a very linear fashion from ~13.2v (no load) to 12.90v (110a load).

 

[OTRwSolar]

@OTRwSolar Please answer the questions in this post.

Large Voltage Drop on BMS Output While Under 110a Load

I'm using a Daly Smart 4S 200a BMS with 4 3.2 200ah cells. With a 110a load the cells are producing 12.90v, however, the output of the BMS is only producing 12.15v. That's a huge .75v drop. The 12.15v reading is being taken on the output of the BMS (same reading on both sides of the BMS...

diysolarforum.com

For bonus points, please avoid prose as I have the attention span of a goldfish .

Tested the voltage differential at the BMS P- and B- (one VM lead on P-, other VM lead on B-) while under a 110a load this morning: .768v. There was little to no voltage drop until current reached a 20-30a threshold. The voltage drop is definitely not linear (vs. increase in current). This reading concurs with the BMS .75v voltage drop I've been observing at 110a (12.90v LFP cell terminal vs. 12.15v BMS output).

I've been using this system with two GC2 lead-acid batteries powering 100-110a loads for two years with zero voltage drop issues (.15v voltage drop at 110a from battery terminals to inverter input terminals---within tolerance for this application). I made no modifications when I dropped in the LFP cells and Daly BMS. Simply bolted up the neg/pos terminals and turned it on.

How thick is the cable from the battery positive all the way back to the battery negative?

Entire run is .46 in. (4/0 cable)

How long is the round trip circuit?

Approx. 20 ft round-trip.

Are there any hot spots?

No hot spots.

Check all the connection points.

No evidence of arcing or hot spots.

Thanks!

 

[John Frum]

Seems like a lot.
What make and model of bms do you have?
If its a smart bms can you test to make sure that the fets can open the charge and discharge paths?
Test with only a few amps to avoid wear and tear.
I wonder if the fets have welded closed.

 

[OTRwSolar]

Seems like a lot.
What make and model of bms do you have?
If its a smart bms can you test to make sure that the fets can open the charge and discharge paths?
Test with only a few amps to avoid wear and tear.
I wonder if the fets have welded closed.

Yes, I agree it's a lot. It's a Daly Smart 4S 200a (no fan; build date 17 Mar 2021). There's a lot of folks using the higher current Daly Smart 150a/200a/250a BMS's. I'm hoping one or two would be willing to do a quick BMS voltage drop check (BMS P- vs. B-) at =>100a to confirm whether this high voltage drop is normal or not.

I don't believe the mosfets have closed. I purposely set the low voltage cutoff very high, pushed 110a, and the discharge mosfets shutdown power completely. Haven't had a chance to check the charge side. Will follow-up with your suggestion to manually turn the charge/discharge mosfets off/on just to confirm.

I've done a number of 30 min. tests at 110a, adjusting operating parameters, checking battery capacity. This BMS does get *very* hot to the touch. Can't imagine how hot it would get at its rated load of 200a. Will P ran 290a through a Daly Smart 250a (with fan) with no obvious issues so that's encouraging. Agree, these tests are a lot of wear and tear, but I'd rather have it fail at home vs. some desolate canyon in the middle of nowhere

Thanks again for your help!

 

[John Frum]

I purposely set the low voltage cutoff very high, pushed 110a, and the discharge mosfets shutdown power completely.

That is what a fet based bms is designed to do but it doing it at serious current causes extreme stress to the fets.
Think of the bms as the reserve parachute.

 

[OTRwSolar]

That is what a fet based bms is designed to do but it doing it at serious current causes extreme stress to the fets.
Think of the bms as the reserve parachute.

Good point. Felt it was prudent to make this one test to set my mind at ease. Unfortunately, I don't have the utmost confidence in this Daly. Have given some thought about carrying a spare BMS with us while we're on the road.

 

[Ampster]

Whatever the loss is caused by it is clear it is happening in the BMS. It amounts to 5% and in my grid interactive installation where I can bank energy at $0.30 to $0.50 that inefficiency can be quantified. If it was happening to me I would get another BMS.

 

[snoobler]

0.768V drop between B- and P-?

V = I * R

R = V / I = .768/110A = 7mΩ

That seems high if you don't have a loose/improperly torqued bolt.

 

[Ampster]

Have given some thought about carrying a spare BMS with us while we're on the road.

That might be prudent since this could be an early sign of some component failing.

 

[OTRwSolar]

Whatever the loss is caused by it is clear it is happening in the BMS. It amounts to 5% and in my grid interactive installation where I can bank energy at $0.30 to $0.50 that inefficiency can be quantified. If it was happening to me I would get another BMS.