Mosfet to "push back" on MPPT allowing outer feedback loop for the BMS? - Page 1
Forum section: electronics / general

author: paulca
author_rank: Super Contributor
timestamp: April 13, 2023, 03:00:36 pm
content: The problem which currently exists in DC panel + MPPT + battery bank scenarios is that virtually none of the charge controllers have any "individual cell awareness". If and when the charge current exceeds the point at which a single high cell will cause the BMS to terminate all charge current there is no way to tell the MPPT to back off.In a combined, "balance charger" the inner feedback loop controls the voltage/current regulation while the second outer feedback loop looks at the cell max voltage and deltas. In "good" MPPT controllers this outer loop exists but only monitors "pack" voltage. In a "balance charger" it has individual unloaded voltage taps on the cells and will control the charge current accordingly.Smart comms and a management component that changes the MPPT config based on digital values of the cell voltages is plausible, but clunky.What if we took a page out of Asus's book and just lied about battery voltage entirely to the MPPT?Lets say the scenario develops where the MPPT is seeing 14.40V and is tapering current. Just not fast enough as the BMS is trying to hold a high cell down at 3.65V. Too much current is coming in, it's going to blow through 3.65V. Let's introduce a 50mV voltage drop in the negative battery line, post BMS. The MPPT's 14.40V actually becomes 14.35V at the battery and the current consumed falls.Not 50mV but an error amp in the BMS based on the max cell voltage and/or the cell delta voltage.Most people actually seem to manage this by under charging the batteries. Never bringing them above 14.0V for a 4S pack. Giving the balancer plenty of time. However this also leads some folks into a false sense of security around the need for balancing in any form. There is a cohort out there that firmly believe 14.0V is the limit and that no balancing is required if you obey that. The same people have never measured a cell voltage in their lives and own harbour freight clamp meters as their only meter. The only thing I think keeping these types safe is the diligence of the battery manufacturers of the LFP 4S pre-made packs they are buying which contain balancing BMSes and they don't even realise it.I'm not interested in arguing with them, but I would like the full outer loop charge feedback to contain the cell voltages somehow.Thoughts? Face palms? Yes, but nos?EDIT: I suppose the first thing I should do is measure the voltage drop across the BMS mosfets during the upper portions of a charge and see if it does in fact already do this. It has got 100A worth of mosfet bridge, what if it doesn't just use it for ON/OFF disconnects, what if it could lower the VGS into the linear region and create that drop.

author: uer166
author_rank: Super Contributor
timestamp: April 13, 2023, 06:23:19 pm
content: Quote from: paulca on April 13, 2023, 03:00:36 pmNever heard of such a problem.. It's only an issue if the battery is totally out of balance and is f*cked, at which point there's nothing wrong with relying on BMS to do its' job and disconnect the battery from charger. I suspect that this is a solution in search of a problem, and the real solution is to ensure that the battery has a proper BMS (whether or not it includes balancing is a separate question, some BMSs don't and that's fine).

author: paulca
author_rank: Super Contributor
timestamp: April 14, 2023, 11:52:54 am
content: Quote from: uer166 on April 13, 2023, 06:23:19 pmIt comes down to the numbers. Maximum charge current versus maximum balance current. Usually resulting in a peak pack charge voltage in order to provide a margin.I'm about to find out, assuming the sun plays ball for another hour or two if by setup and margin with 0.4A balance current can survive at top off which out a disconnect at 22 Amps. I will post data.

author: paulca
author_rank: Super Contributor
timestamp: April 14, 2023, 01:18:04 pm
content: Scratch that. Test aborted.Cell 1, which was apparently the highest cell most of yesterday looked like a real bad lagger today, consistently 40-50mV below it's peers this morning when they got the 22A sun.I could smell a bug, they stink. So I went out to the battery with my multimeter and phone and measured everything myself. Repeatly. 3 cells read within 2mV between my BM235 and the BMS, but cell 1 consistently under-read by 8-10mV (in a cloudy period).What changed since the cell looked fine then? Well, I attempted to to calibrate out that 2mV variance yesterday afternoon. I screwed up.There is a list of readings like:Cell_Resistence_1 0.240mOhm (240uR)There are settings which read:Cell wire connection resistance 1: 0.00I conflated these together "assumed" when set to 0.00 meant "auto" and 0.240mR was it's auto guess. No. Indeed that is not what they do. They literally allow you to input the exact milliohm resistance of your cell voltage tap wires.... such that they are not assumed to be part of the cell resistance. Leaving them at 0 might account for the 2mV deviation, might.However, after setting it first to something sensible like 0.250 (thinking I was forcing a set 0.250 reading on the cell resistance), and seeing no response in that reading, I tried 1.0 and then 3.0 mOhms. Finally concluding that it did nothing.... and critically not setting it back to 0.000mOhm.So, yes I left cell 1 set up to have a 3 milli ohm resistence in it's cell tap wire. That is why it was reading low. Doh!It always pays to go to the actual cells with at least one independent meter. Should anything look suspect.Sun probably won't play ball anyway, but if the chart looks interesting I'll publish the results mistake and all.