Just bought a lot of Tenergy NiMh 2600mAh AA batteries that are going to be used in a hand held vacuum (High drain rate)
What I want to do is protect them from over-discharge. The person/people using the vacuum won’t give a flying feckle about the batteries or that the vacuum motor has slowed down significantly.
Is there a way to limit the discharge on these batteries to 0.9volts or whatever might be a “safe” level?
Some kind of circuit that would just stop the flow of current at a certain voltage? I know they have them for LiIo…but these are 1.2v NiMh.
Essentially, don’t worry about it. It’ll get so weak the vacuum won’t be worth using. Most probable scenario is a dead (zero volt) cell. Put it in a dumb charger a few minutes and it’ll likely be ready for charging again. If it won’t recover, chuck it. Worst scenario is you kill one (some) cell(s). Big deal, it’s a Tenergy 2600mAh, which unfortunately are not very good cells if they are the blue ones, I know, I have a bunch of them….mistake, but not a horrible mistake. Next time get better cells.
How many cells does this vacuum take? Is this a commercial application?
Note: looked them up. Yup, they are the blue ones, too bad. The Tenergy Centura LSD seem to be OK cells……so far. I’ve only got about a year experience with them so my jury is still out.
Thanks for the reply and I know where you’re coming from.
But……
I’d like to skip the value or quality issues of said batteries because I still want to know if it’s doable……
I have another reason for wanting to know if there’s a way to protect 1.2v cells.
I haven’t really bought them yet……but was about to this evening……so I just went ahead and posted that I did for the arguments sake.
I personally don’t know of any NiXX over discharge protection mechanism. Been using them extensively for over a decade, never run across such a device.
If the number of cells is at least 3, you can use a li-ion BMS (leave alone the 1 or 2 remainder cells if not exact multiple of 3, they'll be fine). It may limit a bit the charging voltage, but this is probably no big deal. Certain protection chips allow higher charging voltages, though it is unlikely for a stock BMS to come pre-configured for any higher than 4.2V per stage.
Practically flat is not an issue with NiMh. You can run single cells dead and not do much damage if any. The problem arises when you run a series too hard, one cell goes dead, and you continue to draw current from the ‘pack’. This can cause reverse polarity in the zero-volt cell and can effectively ruin it. I have seen this in multi-cell packs. I have not seen this problem in simple pairs like you are going to use. My experience is one cell dies, you ‘boot’ it in a dumb charger a couple minutes > throw it in a decent charger, and let it charge back up.
IMO, you are over thinking the issue. Doubt the vacuum will run worth a damn on depleted cells if it is a high draw application. Not sure how well those particular cells will work in actual use. Unfortunately HJK does not seem to have done a review on them. My experience with the blue Tenergy cells has been lackluster. I will never buy them again.
Have you considered just buying a few and doing field tests?
I'd just replace that in series pair with a couple high discharge li-ion cells and a BMS. As a bonus, the tool's performance will increase considerably. In fact I did this to my hairclipper months ago and ;-) it rocks!
Over, undervoltage plus overcurrent protection, flydiver. Lots of multi-cell BMSes do not have balancing logic but it can be added with inexpensive boards. Carefully wired multiple BMSes in parallel increase current handling if required, too.
