I got mine from Docan. It was a fun project, but I'm not sure I'd do it again with all of the cheap batteries popping up on Amazon. Not quite as cost efficient. I did build in a heater and fan in the box, which adds protection is pretty cool though! :D
Running microwave shutting down BMS
- Thread starter GetOut
- Start date
It's hard to spend the extra money on cells when you can buy off-the-shelf. But I have a 280 Ah DIY battery in about the same space as a 100 Ah off-the-shelf battery.
Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
The great thing with LFP is you can have different sized battery packs without huge problems. I run 3x 280AH, 2x 174AH & 1x105AH in one bank. Yeah I know what a weird mix, but the 105 is going to the Toolshed so temporarily it's in my main bank. They are all running with JKBMS and the EVE 280's & 105's all came from https://szluyuan.en.alibaba.com/ as did the BMS's which are cheaper there. They carry JK, JBD (Overkill) and others like Seplos, Pace etc.
The GOTCHA ! (of course there has to be some, right)
Everything works totally peachy when all the packs are within the working voltage ranges. 11.6-23.2-46.4 to 13.6-27.2-54.4 which is from 2.900-3.400 Volts per cell.
-- When Charging everything stays nicely together voltage wise right through to 100% SOC (Assuming 100% is 3.400Vpc).
- As lower AH Packs fill and their "Amps Taken" drops below EndAmps/TailCurrent they go into rest/balancing mode taking only minimal voltage while balancing out internally. The larger packs continue to take Amperage until they also get just below their EndAmp/TailCurrent value. When ALL are "saturated" and not taking any amperage, the packs balance out between themselves to the Set Float Voltage.
-- When Discharging, they all stay in-line and discharge Proportianetly relative to their capacity, IE a bank with a 100AH & 280AH with a 30A demand will show as 10A being drawn for the 100AH & 20A from the 280AH which is perfectly normal. This will continue this way until your cells reach the bottom of the working Voltage Range at which point you will start to see deviations between the packs. The lower power packs will discharge less while the larger packs pickup the load, (as expected). BUT the larger packs will also attempt to Backcharge the smaller packs and this will be seen with the BMS as the amperage transfer occurs. At this point there is LOSS of energy as the transfer from one pack to another loses some along the way.
!! DUE TO THIS ISSUE noted above, one must NEVER set BMS Low Volt Disconnect too low otherwise you may not be able to restart a disconnected BMS to take charge if too low. If things have gone too far and the bigger packs back-charged and still dropped below LVD they likely will NOT recover to reconnection point. It is best to set LVD (Low Volt Disconnect) to 2.700 or better yet 2.800 and set the Recovery Point to 2.750 or 2.850. It is also better to keep the reconnect threshold @ 0.050 volts which is well within the recoverable range post demand.
Calculate EndAmps/TailCurrent:
This is a simple calculation that determines at what point the cells are full "saturated". This is the point where Bulk/Absorb Charging has to change to Float (Variable Current which tops off battery packs while servicing ongoing demand as available).
100AH Battery X 0.05 = 5A, or, 280Ah X 0.05 = 14A.
With a Bank of Battery Packs, you calculate what the EndAmps are for all the packs, If for example you have a 100AH & 280AH packs in Parallel, then the EndAmps for the 100AH is 5A and the 280AH is 14A, divide the difference and use that (14A - 5A = 4.5A. 5A (lowest pack) + 4.5A (the difference divided in half) added to lowest pack for 9.5A Bank EndAmps. *Number of Battery Packs in Parallel makes no difference".
BTW Cash Saving Tip: (adds up fast too)
IF buying cells from Vendor A check to see if they also carry the BMS' etc that you need. Most often they DO and if you get them to package it all together, you save on the MIN Cost "per box" + Min Weight charges it adds up fast, especially with different vendors shipping stuff. The Couriers charge "per box" minimum fee, box size & weight fees & number of packages rate... AND ALSO depending on What Country being shipped to (Like the USA) there is a new additional "Consumption Tax" which just came into effect (May 2023).
Hope it helps, Good Luck
Found this thread interesting. Figured Id put this out here.
I built 2x 230Ah using grade A EVE cells each using the Jiabaida 200A with heater ports.
The 2 batteries are next to each other connected in parallel using massive 2/0 pure copper gauge wires, both same lenght.
2/0 gauge (+) lead from battery #1 to the inverter & main circuit with 250A inline fuse, same lenght of wire for the (-) lead connected to the battery #2.
On charging with the onboard converter, bat #1 will draw at least 25-30% more than bat #2. Same thing will happen with discharge under a heavy load.
I figure this setup would equalize draw and discharge to both. But the difference is more than 10%.
Another thing I noticed: When both batteries are fully charge and you introduce a small draw of 1-2 amps, only one battery will supply and gets slowly depleted while the second battery will just stand still. Its like the second battery is on a vacation break or something.
I then shut down the discharge of bat1 thought the BT app, and bat2 will wake up and take over. Re-activate discharge on Bat1, and now both will supply (again, largely unbalanced concerning the outputs)
Im suspecting Jiabaida's BMSs are not completelly "agreeing" to split the bill or the In-line fuse is adding enough resistance to unbalance both main lead wires.
Anyone has any inputs on this? Or clues?
NOTE to the original poster: I have no issues running the inverter (2200W Giantel with only 1 battery) to drive my 3way microwave (+1500w on the inverter's display). It will handle the surge without any issues.
Although, that was after I swapped the 1 gauge leads to 2/0 gauge, AND removed the 175A Anderson connector used as a quick disconnect. Before this, the surge would sometimes trip the inverter into low voltage mode, i suspected the resistance in the Andersone connector was significant enough to do this.
I built 2x 230Ah using grade A EVE cells each using the Jiabaida 200A with heater ports.
The 2 batteries are next to each other connected in parallel using massive 2/0 pure copper gauge wires, both same lenght.
2/0 gauge (+) lead from battery #1 to the inverter & main circuit with 250A inline fuse, same lenght of wire for the (-) lead connected to the battery #2.
On charging with the onboard converter, bat #1 will draw at least 25-30% more than bat #2. Same thing will happen with discharge under a heavy load.
I figure this setup would equalize draw and discharge to both. But the difference is more than 10%.
Another thing I noticed: When both batteries are fully charge and you introduce a small draw of 1-2 amps, only one battery will supply and gets slowly depleted while the second battery will just stand still. Its like the second battery is on a vacation break or something.
I then shut down the discharge of bat1 thought the BT app, and bat2 will wake up and take over. Re-activate discharge on Bat1, and now both will supply (again, largely unbalanced concerning the outputs)
Im suspecting Jiabaida's BMSs are not completelly "agreeing" to split the bill or the In-line fuse is adding enough resistance to unbalance both main lead wires.
Anyone has any inputs on this? Or clues?
NOTE to the original poster: I have no issues running the inverter (2200W Giantel with only 1 battery) to drive my 3way microwave (+1500w on the inverter's display). It will handle the surge without any issues.
Although, that was after I swapped the 1 gauge leads to 2/0 gauge, AND removed the 175A Anderson connector used as a quick disconnect. Before this, the surge would sometimes trip the inverter into low voltage mode, i suspected the resistance in the Andersone connector was significant enough to do this.
Check the connections. When discharging or charging hard, is there a difference in temperature of the connections.
Are your batteries interconnected? (cable between the batteries) If so, consider running cables directly from each battery to the common bus bars instead of between the batteries. You'll need four equal length cables (the BMS cable(s) counts as one cable from the negative side of the battery).
It could be a problem with the BMS. My two JBD BMS are very close to the same amps when charging/discharging.
Tulex
Solar Wizard
One more thing to love about this place, never know when info is going to come your way that you didn't even know you needed. I had no idea these even existed. It's always bothered me how microwaves work for lower power. Will be in the market for a new one in the next year, now I am more informed.
Tulex
Solar Wizard
That was a very common misconception from the very beginning of microwaves, that they heat from the inside out.
Actually, the same thing will happen on a small load. but worth a try. What I could do is switch the parallel connection cables. maybe one of them has a not-so-good crimp on the lug connector.
yes, interconnected with 2x short 2/0 gauge leads.
Looking at the proper way to parallel connect using copper busbars, the way I have it should theoretically work. Don't get me wrong, Im would be ready to try what you suggest (running 4 leads from batteries to the inverter), but I would have to purchase another 4' of 2/0 gauge wires @ 9-10$ a foot. I would rather investigate other solutions prior to spending another +40$ for a trial.
Internally, the negative wire from the banks to the BMS 'should' be equal, but I did build those batteries at different times. You just put a doubt in my head. will have to check that!
A couple of inches of difference in cable lengths isn't going to matter. Different cables (3 x 8 gauge vs 2 x 10 gauge) will make a difference.
Ampster
Renewable Energy Hobbyist
I am obviously a fan of a single pack. Mine is 3P16S because I do not want to deal with the issues expressed earlier.
Tomthumb62
Solar Wizard
- Joined
- Sep 25, 2022
- Messages
- 1,871
What are your dimensions.
Our of the shelf 200Ah is 19 inches long, 8 inches tall and 9 wide. It’s HUGE. Takes up a lot of space in our tiny trailer.
About 13" long, 9" tall and 7" deep. The BMS is attach at the end, so add another 1" to the length.
Help! Please educate me… I have 2 *. 260aHr LiFePO4 batteries (I believe it was supposed to be a 125 amp BMS on both of them.) They are wired in parallel for a total capacity of 520 aHr storage. That said, does this mean I only have 125 amps available before throttling or is it actually double that with them wired in parallel and actually 250 a before it’s throttled?
I’m curious, as I tried running our microwave from the inverter while parked and the MW would start but after a few seconds it just shut down … And hence, I wasn’t sure if it was the BMS or the inverter causing the issue… how do I determine which is the cause of the problem? We’re running the MW off the 3000 watt PSW (Renogy) inverter. Tia!)
No, you have twice what the BMS is rated for. If each BMS is rated for 125 amps then the total is 250 amps. That assumes that each battery can provide 50% of the requested amps.
Can you provide links to your microwave, inverter and battery/BMS? Let's see how much juice the microwave is like pulling and how much your system is capable of providing.
A picture of how your inverter and battery are wired, along with the specs of the wires, will help.
A 3000 watt inverter should be enough to run a microwave in an RV. However, if you had other loads that were competing for amps then I could see where the inverter and maybe the BMS would shut down.
Tomthumb62
Solar Wizard
- Joined
- Sep 25, 2022
- Messages
- 1,871
How many watts is your microwave?
According to a quick google search, a 900W microwave can draw as many as 2700W to start up. This means it can draw about 250A from your battery for 1-2 seconds. A decent BMS should be able to handle a short burst like that, and even then your 2x125A BMS is good for 250A, continuous...in theory. Also your wiring can make a huge difference. If your wiring between the batteries or between the inverter and battery bank is too small, you WILL have voltage drop and the result is the microwave will try to draw even more amps to make up for it, resulting in more voltage drop...it can become a vicious cycle.
So as @HRTKD suggested, a photo of your wiring would help people give you more specific advice.
I’ve included what is noted on the MW for the power rating in the one attachment. (All I recall is it’s noted to be a 2002 model and LG but apparently rebranded as Dometic.) Not sure what the wiring is for the MW because it was pre wired as well as unsure what the wiring was that they used to pre wire it for the solar. It’s a 2003 Arctic Fox 990. The wires for the batteries and the inverter are using 2/0 copper wires. The batteries were at least 97% when we attempted to the MW yesterday when it kept cutting out… yet it’s odd as I’m pretty sure we had it work once before earlier this trip. Also just tried the MW again in the driveway and it’s still shutting off after 2-3 seconds and was at 98%….I just took a picture of the microwave specs inside the MW and also included a picture from the manual I found online as attached. (When I put the KillaWatt display meter on tonight, it read 1380 watts and 13 amps and didn’t seem to catch any sort of surge at startup.) ps- Sadly I was not able to get any good shots of the setup at this time.
Attachments
Conservatively, the microwave is going to draw 135 amps on the DC side (1380 watts / 12 / .85 = 135) based on your Killawatt meter. The label says 13 amps AC and using that we get 152 amps DC, Which should be well within the capability of your two batteries.
It would help to know what is shutting down. Was it the inverter or one of the batteries? You'll have to look at the logs. If even one of those batteries shuts down then you lose everything on the DC side because the other battery will also shut down due to overload from trying to carry all those amps by itself.
Check all your connections. A poor connection can show great voltage, but when put under load you'll get very few amps and the voltage will drop.
For comparison, my microwave is very similar.
My inverter is a Multiplus 12/3000 (effectively 2400 watts). I've had no problems running my microwave on high for five minutes. Running continuously doesn't sound like your problem, it's the startup surge. Before blaming your inverter, we need to know if your batteries are shutting down.
It would help to know what is shutting down. Was it the inverter or one of the batteries? You'll have to look at the logs. If even one of those batteries shuts down then you lose everything on the DC side because the other battery will also shut down due to overload from trying to carry all those amps by itself.
Check all your connections. A poor connection can show great voltage, but when put under load you'll get very few amps and the voltage will drop.
For comparison, my microwave is very similar.
My inverter is a Multiplus 12/3000 (effectively 2400 watts). I've had no problems running my microwave on high for five minutes. Running continuously doesn't sound like your problem, it's the startup surge. Before blaming your inverter, we need to know if your batteries are shutting down.
Tomthumb62
Solar Wizard
- Joined
- Sep 25, 2022
- Messages
- 1,871
So your microwave lists a consumption of 1500W. So let's call it a 1500W microwave, even though it only outputs 1000W.
1500w/12.8v/0.85 (inverter inefficiency) = 137A.
That 137A is the running power, but the startup surge would be more.
Let's say startup surge is 4500W. Your 3000W inverter is supposed to be able to handle twice that, for 6000W. But for how long? Not sure Renogy lists for how many seconds (or milliseconds is probably the likelihood) it can handle 6000W. Or how many milliseconds it can handle 4500W for your microwave startup surge.
But it might be your startup surge amps is more than the BMS's can handle. 4500w/12.8v/0.85= 413A
Again, your BMS's, the two together in parallel, can handle up to 250A continuously, but how much can they handle for surges and for how many milliseconds, before the BMS shuts down?
Perhaps someone can help you narrow down if it's the inverter or the BMS konking out, but my bet it's one of those.
I have also heard that microwaves can be especially difficult for some inverters to start because 1) surge is actually 5-6x, not just 3x (clamp meter could measure this, but you'd want a beefy one) and/or 2) something to do with microwaves being out of phase (this is where my electrical knowledge plummets so I'm parroting from poor memory) with the inverter (not sure what the fix is).
Unfortunately I really can’t get any good pictures as it’s pretty tight in there ….the wiring is located in the cupboard that also houses the sink plumbing (and yes, I know, electronics and water is a bad combination… but it was really the only place there was available for it as we’re in a truck camper and there’s very limited available real estate available…)
Just need to know how to determine if it’s the bms or the inverter that’s causing it to cut out at startup.
