Author Topic: Internal Resistance Calculator (Read 14052 times)

AzPlumber · « **on:** August 16, 2014, 02:46:04 PM »

Anyone know of a online calculator for internal resistance of a cell? It would be nice to just plug in a few numbers and have the math done for you. Seems there are calculators for everything else but can't find one for this.

CraigHB · « **Reply #1 on:** August 16, 2014, 03:21:09 PM »

I don't know that's something really justifies a calculator. In terms of DC resistance (DCR), you can just put a known load on a cell and measure how much voltage drops, then divide voltage drop by current flow to find DCR. In terms of the math it's simply a fraction.

Most data sheets for cells give a number for "internal impedance" which is a bit more involved. That accounts for the AC effects of the load on a battery, but it's typically pretty close to DCR. It can give you a good idea about a cell's DCR in the least.

Another way to find DCR is to look at the cells discharge curves which are sometimes posted in the data sheets. You can take the difference between current and voltage from one curve to the next then divide them to find DCR. For example, V₂-V₁ / I₂-I₁

AzPlumber · « **Reply #2 on:** August 16, 2014, 04:26:17 PM »

Quote from: CraigHB on August 16, 2014, 03:21:09 PM

I don't know that's something really justifies a calculator. In terms of DC resistance (DCR), you can just put a known load on a cell and measure how much voltage drops, then divide voltage drop by current flow to find DCR. In terms of the math it's simply a fraction.

Thanks for the reply Craig. This is what I'm having a hard time understanding (I'm a plumber). If I have a one amp current draw and let's say a drop in voltage of .2 do I just divide .2 by 1 ? If so, does this give me a DCR of 200 milliohms

CraigHB · « **Reply #3 on:** August 16, 2014, 04:40:27 PM »

Yes, that's correct.

One thing about Li-Ion cells is they do some rather odd things with voltage. All batteries exhibit these characteristics, but Li-Ion cells do it in a more pronounced way. First they have something called voltage creep. That's where voltage will slowly rise after a load is removed. Voltage slowly drops after a charging current is removed. They also change voltage with temperature. Then the amount of voltage sag varies depending on the state of charge. All of these factors can make it difficult to nail down a battery's DCR by taking measurements on the bench.

When you do voltage drop checks on the bench, use a median charge at room temperature. Do a short discharge on the cell first to remove the voltage creep. Then measure quickly the difference in open loop voltage and loaded voltage. Divide the current flow into that difference to get DCR. Of course you need to know current flow so use a calibrated resistance or an ammeter.

You'll get a median value for DCR that way, but keep in mind it can as much as double under the extremes of temperature, load, and state of charge.

AzPlumber · « **Reply #4 on:** August 16, 2014, 04:52:03 PM »

Thank you, just wanted to make sure I was on the right tract. Sometimes I tend to over think this electrical stuff I guess. I'd rather ask the question and not just assume I understand.

CraigHB · « **Reply #5 on:** August 16, 2014, 05:37:51 PM »

Welcome,

DCR is often a neglected characteristic of batteries that can have a big impact on performance. It's good to see that becoming of bigger interest with mod builders. It also has safety implications. Higher DCR means more heat and you don't want to get a Li-Ion battery too hot.

AzPlumber · « **Reply #6 on:** August 16, 2014, 09:38:17 PM »

The heat is exactly why I'm interested in the DCR. Here in the Valley of the Sun it gets pretty dam hot. The temperature inside a vehicle can reach 140 and a metal mod in direct sunlight can get too hot to handle. Trying to avoid the heat damaging batteries is challenging but I do what I can. Hopefully I can avoid a battery melt down by keeping an eye on their DCR.

CraigHB · « **Reply #7 on:** August 17, 2014, 01:46:40 AM »

Li-Ion batteries have an operating temperature limit about 140F so if you leave one in a parked car you can get it too hot. If you're not loading the battery too heavily and it's a high drain battery (they have lower DCR), it shouldn't heat up much additionally under load. Might be a good idea to bring the mod inside with you when the temps are high.

Years ago I spent some time in Yuma during the summer for a contract job and that was the hottest environment I've ever been in. Pretty much the same as Death Valley. It can get so hot in the car when left parked, plastic stuff left inside can melt. If you forget to roll down the windows, things can get damaged. Definitely could destroy a mod in that heat.

My mods have a black box and I left one in the sun on a hot day recently. I think it got to 160F, was too hot to hold when I picked it up. I let it cool off in the shade before using it. Li-Ion batteries actually have to get pretty stinking hot before they'll have a problem, but if they start out hot, they can climb too high under use.