Checking Multimeter Calibration at Home
- Thread starter WheezyRider
- Start date
Thanks @WheezyRider for starting this thread, @StuartsProjects for bringing the TL431 voltage reference board to my attention, and everyone else for their contributions - I now know this is a reasonably accurate voltage reading of my 36V battery charger's output. YVMV (Your Voltage May Vary):
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me too, I appreciate the usefullness of this thread, and for the links to various voltage reference boards that you can buy.
However, if your charger works until now, there is no need to worry that you need to buy additional test equipment, DVMs and reference boards. BMS and charger are designed with a degree of tolerance. Your BMS will always do the balancing job at the end of a full charge, even if the drift is as much as 0.5V
However, if your charger works until now, there is no need to worry that you need to buy additional test equipment, DVMs and reference boards. BMS and charger are designed with a degree of tolerance. Your BMS will always do the balancing job at the end of a full charge, even if the drift is as much as 0.5V
Glad people have found this thread useful.
If your charger/battery combo works now, that's good. But, if it is running at 41.8 V or less, it will not balance the pack in most cases. If it is giving out 42.5 V...maybe that's ok - there could be diode in the charge circuit, so the charger needs to compensate for the diode drop. If there is no diode and the BMS is exposed to the full 42.5 V, it probably has enough slack to be able to absorb that, but it's likely that the cells will be stressed at their upper limit (towards 4.25 V/cell), shortening pack life. Indeed, if one or more banks is out of balance, the best banks in the rest of the pack will be pushed well above 4.2 V/cell.
For chargers giving out more than 42.5 V? At what point does the BMS fail? At what point does it become dangerous? This is particularly important to think about if you buy a replacement or a second charger online, or from a different supplier to that of the battery. I've bought 42 V chargers online before that were giving out over 43 V.
As we have seen, some cheap DVMs are very good, but others I've seen are more than 1 V out on the 200 V range. Checking such a charger with an inaccurate DVM could provide a false sense of security. So having some form of simple calibration is not a bad idea, even if it isn't perfect.
yes, I keep repeating this, balancing happens at the end of full charge, when the charging current goes below a set level, typically 100mA. That's the reason why bleed current is less than 100mA.
there is no truly safe voltage for charging/discharging. The level of danger increases quickly with voltage but to appreciate that, you need to understand how the voltage is created in the first place.
The guy in that first video hasn't a clue. A normal top-balancing BMS that we have in our batteries does indeed balance the battery if you use it correctly. It wouldn't surprise me if his one is so far out because he's been reading all that BS about 80% charging. He says that the balancing doesn't work because it doesn't charge the low cells. Of course it doesn't charge the low ones. It works by draining the high ones, but it can only do that if you fully charge the battery.
He didn't need to go to all that trouble of manual balancing. All he needed to do was leave it charging and it would have sorted itself out.
A large battery like his can take days to balance passively and it may even not possible because the battery may be needed again before the balancing is completed. That's probably why it is in the state it is, what the guy has done on the video is probably the only way to save it.
active balancing is at least 20 times faster than passive balancing and can solve problems like this without manual intervention.
active balancing is at least 20 times faster than passive balancing and can solve problems like this without manual intervention.
I won't know if my BMS responds with balancing, with the pack being charged by the continuous oscillation between 42.1V and 42.2V, until I get around to measuring the cell groups post-attempted full balance charge... but it's fully charged voltage doesn't seem unusual given it's age and mileage, so I'll probably simply leave it alone.
I'm starting to wonder about whether it'd be wise to cut and embed into/bond onto the battery case a set of Anderson connectors or something and place the BMS externally - that way, I could easily use different BMSs or active balancers without having to do more soldering on the battery pack.
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Do you mean 42.1 and 42.2?
I'm sure your pack is fine. If you are really worried you could get a smart BMS with bluetooth or whatever and see what the banks are doing in real time. Having an external swappable BMS is only going to introduce more potential problems IMO.
Cheers, corrected (middle vision is off reading the screen sometimes).
Especially with my soldering!
I think it might be interesting for an experimental pack, but not your main every day pack that needs to be relied upon to work and be safe.
Those cell holders did look 3D printed. Are you going to print a case to contain that battery pack plus controller?
Yes. We'll start with designs next week. That meter is for the kid I'm training. His homework this week is to put in all the resistors. He has to learn the colour codes before he can do that. They're all high accuracy 5 band ones, so not as easy as most.
Compartment over the controller to keep a Cornish pasty and/or pizza slices warm? I was considering making a waterproof battery hood using aluminium sheet, pop rivets and 3M VHB tape:
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