Resistance of single cell vs battery pack?

[piechockidocent9]

Hello, I've done inventory of all my battery powered tools, discarded quite a few really old, barely functioning ones and I'm ordering replacements. For the sake of testing new arrivals and sending them pack if they turn out to be duds, how do you calculate proper resistance a battery pack should have?
If ok resistance of single nimh cell is 30mΩ -100mΩ then battery pack of 6 cells should be same or does it stack and resistance of such a pack should be 180Ω-600Ω?

 

[Ralston18]

Appears to be a homework like question....

No way to truly know from this end.

Add more details and explain the 180-600Ω solution that you calculated.

How is the internal resistance being obtained or measured.

Show more work and cite references etc, as necessary.

 

[piechockidocent9]

Heh no it's not a homework though my level of knowledge is certainly elementary or worse.

Searching for answers a statement that single, new nimh cell resistance should oscillate around 30Ω to100Ω is the most common, repeated one with some outliers saying high drain cells should have even lower up to 20Ω max. However that's always about a single cell and what about a pack made of 4-6 of those cells?

As to why I'd like to know for example I have Bosch PSR 1440 screwdriver from 2004 year with two factory 14,4v, 4/5cs, 1,2ah, nimh batteries.

Using imax b6 mini charger (grabbing +/- with red/black clamps) instead of factory one, after charging them few times battery meter shows they go to around 16,4v and resistance meter says 190Ω-200Ω.
I know these factory ones are almost dead because straight from charger I'm able to put 2-3 screws in and that's it, back to the charger they must go.

Now I ordered a replacement, once it arrives I'm going to charge and measure it. What value would determine the pack to be in a good state? I assume that if new battery pack will have similar resistance to almost dead ones I have then it's a dud and I should send it back.

 

[kerberos_20]

if youre using multimeter to see resistance, then youre not measuring internal resistance, as there is resistance comming from probes and shunt resistor in your meter

just charge it and check voltage, if voltage too low, replace

 

[Grobe]

I once had a lab on school about this subject. Some key points:

• A battery does not have one fixed internal resistance. The battery internal resistance will vary quite a bit dependent on the load, temperature, battery health and probably other factors as well.
• The voltage drop in a battery as function of current draw is not linear, therefore using voltage drop between no-load and on-load with known current draw won't give a correct result. A small change in current is preferable, but then the volt meter have to be precise.
• Instead - use a method of two equations - use two set of load situations, both under the battery normal load condition (i.e. don't push the battery limit for this) - and you will be able to use this delta voltage drop vs. delta current to calculate the internal resistance under normal condition.
• The internal resistance under a short circuit is normally not tested directly because danger of sparks and also it can damage the battery itself and shortening it's life time.

Hope this helps.

 

[kanewolf]

Have you read this article -- https://batteryuniversity.com/article/bu-802a-how-does-rising-internal-resistance-affect-performance

 

[BFG-9000]

It's not resistance at rest that matters anyway but resistance when under heavy load so you may as well just measure voltage drop instead. Tools can easily draw higher loads than the cell rating so this matters quite a lot for generating heat. And NiMH don't hold up to abusive use very well, which is why most power tools jumped straight from NiCd to Li-ion. Simple NiCd tool chargers also terminate on heat sensing, by which point the NiMH cell will already be damaged. And if you have to change the charger anyway, may as well convert to Li-Ion instead.

Surely you've heard of the new tabless cylindrical cells that greatly reduce resistance by providing much more surface area for attachment than the usual couple of tiny spotwelds. Tesla and Ryobi have gone for those. Ryobi packs were of course originally designed to hold 15 sub-C cells including one in the neck so much of their internal space is wasted today.

The other way of doing something similar is with pouch cells where there is plenty of room to put connections across an entire edge. FLEX, Dewalt and now Milwaukee have gone for those. Pouch cells must of course be constrained from puffing up or the usable surface area within the cell will be greatly reduced, leading to reduced capacity and hotspots which could cause a fire. RC hobbyists are of course infamous for abusing li-po pouch cells until they often catch fire, but apparently safer chemistries have been developed for power tools