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Forum Index : Electronics : Fuses & Breakers for use with battery banks.
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Revlac Guru  Joined: 31/12/2016 Location: AustraliaPosts: 1171 |
Instead of dumping on someones thread I will fire up on this one.  I have been watching a lot of solar Inverter battery installations in various places, often some fascinating stuff and some good ideas, some not so good. Its always good and recommended to have a fuse (or breaker) between the battery and inverter charge controller etc, sizes correctly to protect the wire from burning (in an overload event) or smaller for some reason. Now I have read many times in many places about fusing each battery bank when connected in parallel, this is also recommended and for the same reason as stated, now there are many comments about the fuses been there to protect a shorted cell (in a battery bank) from getting a heap current from a parallel battery bank, I have had my doubt's for a long time, about this working, only under the right circumstances will it actually blow the fuse and by that time its likely already a disaster. With a partially shorted cell, it may be a few amps or perhaps many more but still less than the rating of the fuse and will not blow the fuse, however the voltage would fall on that particular shorted cell and the other cells would be forced higher from either the other parallel batteries or charge controller (if charging) in this scenario the BMS would see over voltage on other cells and shut down charging for the battery, or if the BMS is setup or equipped with cell differential cut off it would protect that battery bank from the parallel banks dumping power into it. Now if the BMS has failed and can't shut off power to that battery with the shorted cell, it may eventually blow the fuse but you have a sh!t show on your hands. I have used a car headlight wired to a cell that had voltage a bit higher than the others (the old crappy cells in the shed I use and experiment with) and left it on longer than I should, other cells raised voltage and BMS shut off, very simple just charged that cell back up and all is running well again. I just don't expect a fuse to save the battery bank if there is a shorted cell... Its not in the fuses job description, Keep the fuse in place for its primary role. Just my view on this, others might see it differently, there is likely a safer more reliable way of dealing with a shorted cell, even if such a rear event...ideas? I have had a partially shorted cell in the FLA battery bank and it was gassing off the other cells due to over voltage, nothing to stop that, just user intervention required....too bad if your not home to fix it. Edited 2025-09-01 19:49 by Revlac Cheers Aaron Off The Grid |
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| Godoh Guru Joined: 26/09/2020 Location: AustraliaPosts: 559 |
I get what you are saying Aaron. I don't have experience with large lithium batteries except in our car. I have only ever had AGM, SLA and FLA batteries on my off grid solar systems. My experience tells me that the internal resistance of the batteries is most likely too high to protect a parallel bank as you suggest. I do have fuses on the output of the battery bank, and also use circuit breakers on the input to my inverters. Having seen enough mosfets self destruct, I figure that with a lot of luck the breaker will trip in the event the inverters turn into smoke. I don't run parallel batteries, just series banks pete |
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| Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1171 |
Hi Pete, Yeah, I have run FLA in parallel many times, also 2 tractors and the truck run 12v batteries in parallel, A real eye opener was a mates truck that had 6 or 8 batteries in parallel, though he did run a small electric hydraulic crane as well. I haven't yet run parallel LFP but contemplating doing it, so will be using fuses on each bank, I have some fuses and parts but not all of yet, no hurry there is 2 inverters running each on its own battery and will soon have one inverter so it makes sense to parallel the 2 banks. Yes agree, Or someone drops a spanner across the bus bars or something silly...these things do happen. Cheers Aaron Off The Grid |
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KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1900 |
Hi Aaron, as you already know, I use a separate fuse for each parallel battery bank. For me, this is a worst case "non electronic" fail safe in the event that one battery bank may decide to draw high current from the other banks, for whatever reason. The 80A fuses I use WILL also blow before the bus feed cables melt or catch fire from any short on the Battery feed cables, and a large reasonable quality fuse will not blow under short duration 200A surges, the four battery banks can supply an 800A surge via 4 x 80A fuses. Some BMS will trip if not set to allow for high 200A maximum output current, so a sustained > 100A current path into or out of one Bank will not trip some BMS, but will in a short time, open an 80A fuse. While you can also set the maximum charge current on most BMS, this only works as long as the BMS is working correctly. Another thing to keep in mind with some parallel Battery BMS banks, the BMS can be self reset if one BMS happens to trip the charge or discharge FETS or Relays, the BMS might switch back on due to 53v across the terminals from the other banks, this is different from under voltage protection where the BMS should wait until the terminal voltage is completely removed before responding to Terminal voltage being restored to re-enable the BMS, but again in all cases we are relying on the electronic BMS devices to be functioning correctly with all user settings intact, some do for 96% of the time, it's that 4% I wish to reduce slightly.  One can only try.NANO Inverter: Full download - Only Hex Ver 8.1Ks |
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| Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1171 |
I'm pretty sure for the most part the BMS will be the first to see a cell short, it (or how) it is set to deal with it is another story, At the moment the Daly BMS has differential voltage protect default 0.80v which is probably quite high considering it would take a lot of current for that voltage change....or a bad terminal connection, I have watched a cell short in my old 18650 battery setup and it would gradually draw down its parallel neighbours (they were parallel cells, ( I Don't use these anymore) it took days to weeks for the imbalance to make a noticeable change. We can charge these LFP cells with quite a high amperage and practically very little temperature change, but an internal short anywhere between 5 and 50Amp will make a noticeable amount of heat and not yet enough to blow a large fuse. I have tripped the BMS and will explain that in the inverter thread, it was interesting.  The charge and discharge on each BMS are still set at 200Amp each, which is as expected for a single series battery bank, I will change that If I parallel the 2 banks, I also have a JBD BMS on the scooter and looking through all the setting I couldn't find any voltage differential settings, it might be there under a different heading or hidden and not user accessible. On another note, all this will be sort of obsolete and perhaps no need for parallel banks when the new 628Ah and 1175Ah cells are available. Cheers Aaron Off The Grid |
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| KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1900 |
All true but, I'm not talking about inter cell OV/UV conditions, these are the domain of the BMS and are normally never seen by the parallel banks until one whole bank turns to #hit. I'm taking about an unexpected short in any bank, that includes the BMS, internal interconnect bus bars and wiring to the main output paralleled connections bus feed. I'm hoping to get a good 6 to 7 years out of the current cells, so those big cells will likely be even bigger if I'm still around when these need replacing.  NANO Inverter: Full download - Only Hex Ver 8.1Ks |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1171 |
I have seen commercial 48v batteries with the BMS completely destroyed, looking like a charred mess, they have taken out the battery main cables and cell monitoring wires all inside the sealed battery case. It only takes a voltage spike to short out the mosfets in a BMS, especially those in sealed up batteries where cost cutting has resulted in use of marginal rated components. You cannot rely on a BMS to protect a battery, there has to be a fuse or breaker directly on every main battery output, before the BMS and any other loads. Two days ago a PV charge controller (new design) that I have been playing with, went bang, the main buck mosfet shorted, placing 2KW 120v PV array directly across the Lifepo4 battery. I had used cheaper Chinese brand mosfets that supposedly had a 220v rating, after replacing with a IRFP4668 US brand (200v rating) its been working fine. If there was a BMS on this test battery, it would have prevented any overcharge condition, but then it probably would also have blown up with the high voltage across it. You always have to have a fail-safe plan "B", if the battery voltage rises to a dangerous value, an independent voltage monitor can open a vacuum relay connecting to the input of your PV controllers. Its all good fun... Cheers Mike |
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| KeepIS Guru Joined: 13/10/2014 Location: AustraliaPosts: 1900 |
 I've seen so many videos and reports of BMS protected big batteries turning to $h!^ over the years, the LiFePO4 cells in most don't appear to combust, or take a lot of abuse before they let go, usually user error, incorrect or poor construction and cost cutting shortcuts, although BMS design and components appears to be getting better, Murphy seem to hide everywhere. NANO Inverter: Full download - Only Hex Ver 8.1Ks |
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| Solar Mike Guru Joined: 08/02/2015 Location: New ZealandPosts: 1171 |
The last 48v 100AH sealed Lifepo4 battery I pulled apart had no insulation between the cells other than the thin blue shrink wrap coating on each cell and they were just held together with a stranded sticky tape of sorts; the terminals were welded. So this was a disaster just waiting to happen and it did, the cells could flex with movement, the terminals could not, so they disconnected somewhat. There is a message here, do not use sealed battery packs, you cannot see what price cutting shortcuts have been made, and it could burn your house down. |
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| Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1171 |
I just put insulation sheets between all the cells in my old setup, years ago when I built it there was very little info about separators between them, I did check and although there was the smallest amount of scuffing there was no damage yet, but given enough time in a stationery setting it would eventually get through that thin blue plastic, much faster for those in a mobile situation, and yes seen a few photos how they turned out. I do have one sealed 24v battery in the scooter, tempted to open that up one day. Cheers Aaron Off The Grid |
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