What is a "suitable resistance" for balancing battery packs?

[jordanbigel]

So, I just placed an order for 3 of these Electric Car Parts Company 400ah packs. They are coming from Utah and should be here in a couple of weeks.

They are 12v/400ah packs which have built in BMS, but they are not "drop-ins" - they has external relays to control charging/loads, relays to turn on/off heaters, etc. They also have a built-in display with data like voltage, current+/-, etc. which is nice because it should help to complete the balancing which is the subject of this post.

https://www.electriccarpartscompany....-battery-packs

I've been working out how to install them and wire the BMS relays, all good there, but now I am reading their guide for how to parallel these packs into a bank, and it's more complicated than I expected. Surprise!

The jist of it is that they want me to parallel them first with some kind of "suitable resistance" (their words) without connecting them to the boat so that they can all come to the same voltage (balance? -- I don't want to confuse people, I am not talking about balancing a set of cells here, although from what I can tell the principle seems to be the same).

I've copied key parts of their instructions below (you can read the entire document online using the link below) - and I basically understand the concept, but it's the mechanics of it which I am missing, what exactly is this "suitable resistance"? Where do I get one? My understanding of things like "resistors" is weak at best - I get that they reduce current and convert it to heat.

Obviously they must also prevent too much current from rushing from one pack to the other, according to the guide "we want to limit balancing current among packs to be no more than 60A, the "suitable resistance" should be "0.3V/60A=0.005ohm" - but my attempts to find such a thing (a 0.005ohm resistor) have only led me to these tiny resistors used on circuit boards, they can't possible expect me to connect one of those between the battery + and a 1/0 cable? How would you even connect that?

I did find some aluminum "wire wound" resistors on Amazon which say they are 0.01ohm (the lowest number of ohms I could find) and it has nice, fat wire leads on it which I could crimp a terminal ring. By this math, if we substitute the 0.01ohm value into the formula, you get 0.3/30a=0.01ohm, which I guess means that if there is a 3 tenth difference in voltage, the max current would be 30amps, using the 0.01ohm resister. Seems to me like this would do the trick, albeit slightly slower (since less current can pass through)?

0.01ohm resistor:
https://www.amazon.com/gp/product/B0...sc_act_title_1

I would really appreciate any guidance on this. Am I even on the right track?

Also, should I even use the cables on my boat? I could probably do this using lighter gauge wire than I normally use, which would make it easier probably to connect the cables to the resistor, maybe 12awg wire since it only has to support 30amp over a distance of about 2ft. Right? That way I could set the 3 of them up in a storage room I have and let them balance overnight (I have no idea how long this could take, so if I remove my current batteries and put these in and start "balancing" I will be without power until its done, which is bad because when there are no lights the Admiral goes to a hotel), and then install them the next day once they are stable and equalized. Does this make sense?

Thanks in advance!
Jordan.



BestGo Parallel Configuration Guide
https://www.electriccarpartscompany....anual-7-19.pdf

• connect the negative poles of battery packs together.
• Then, each pack should have a "suitable resistance" fixed to positive pole, after that, use power cables to connect those "suitable resistances" for parallel config of those battery packs. 

(Explanation: Since packs have different voltages, it will cause the current rush from higher volt packs to lower volt packs, so "suitable resistance" on each pack can control this electricity current rush within an acceptable volume.)

• For example, if one pack has voltage of 51.5V, and the second pack has voltage of 51.2V, there is a 0.3V difference, if we want to limit balancing current among packs to be no more than 60A, the "suitable resistance" should be "0.3V/60A=0.005ohm", or say "suitable resistance" should be no less than 5 milliohm, that 5 milliohm resistance is enough for all of preferred battery pack if volt difference is no more than 0.3V.
For 0.6V difference, please use resistor of bigger than 10 milliohm.
For 0.9V difference, please use resistor of bigger than 15 milliohm.

• After that config, current will pass among packs to equalize the voltage, with the lower one raising and the higher one lowering. Eventually, all packs will have the exact same voltage. Once this is achieved, please remove those “suitable resistance”, and use a standard large cable to connect the positive poles together (please review the pictures in next page for the reference).


<This diagram is part of the guide>

[akopac]

I’m assuming most of the bank is usable. That’s a lot of battery… 1200amphrs?

[HeywoodJ]

Yes, what you linked is “suitable”. More than enough power capability. Using the manual examples 60A @ 0.3V would dissipate 18W for a short time, then drop off as the voltage equalized. At 0.01Ohm you’d drop the current to 30A and power to 9W, that one could handle a 1V difference.

Easier though, is just using a longer piece of moderately sized wire. 10AWG copper wire has a resistance of 1 Ohm/1000 feet, so, again using the example, a 5’ length would present about the right resistance. Yes, it is over the ampacity rating of the wire, but it will be short duration in free air, so I would be happy with 10AWG. 8AWG has about 2/3 the resistance, so a bit longer piece of that for peace of mind. I’d actually probably start with 10-15’ of 10AWG, that will reduce the current and still get the job done.

Either one works, whichever makes you more comfortable.

[jordanbigel]

Originally Posted by akopac

I’m assuming most of the bank is usable. That’s a lot of battery… 1200amphrs?

I'm assuming that it is about 80%+ "usable", but in practice I will likely make due with a fraction of that. I don't use AC at anchor, but I do have a huge freezer and fridge and cruise in the tropics so I can easily use 200ah a day, plus I only have about 400watt solar, and I do not want to turn my boat into a solar farm, so, that's all there is going to be. I also have a small 3kw genset.

My thinking is -- a) the more AH the longer between generator running, b) I like the peace of mind knowing that if one of them fails, the other 2 will provide ample power - plus I have a rack built to hold 4 x 8D batteries, so I have the space available.

[jordanbigel]

Originally Posted by HeywoodJ

Easier though, is just using a longer piece of moderately sized wire. 10AWG copper wire has a resistance of 1 Ohm/1000 feet, so, again using the example, a 5’ length would present about the right resistance. Yes, it is over the ampacity rating of the wire, but it will be short duration in free air, so I would be happy with 10AWG. 8AWG has about 2/3 the resistance, so a bit longer piece of that for peace of mind. I’d actually probably start with 10-15’ of 10AWG, that will reduce the current and still get the job done.

Either one works, whichever makes you more comfortable.

Fascinating idea! I had not though of that - it's so simple, I wish they had just suggested that, could have saved me a lot of wasted time and energy...

Pretty sure I have some 10awg lying about, and I think 10' would be pretty safe, for each connection, so I would need in total 60' of it, hmmmm.

It's starting to sound a bit cumbersome with all that wire, the resistors might actually be easier to manage. Going to think on it, see how much wire I really have on hand. I have some time before they arrive so no rush.

Thanks again!

[HeywoodJ]

You only really need 20’ If you put half on the positive and half on the negative connection you can use 5’ pieces. Two pieces from left battery to center battery, two more from right battery, 20’ total gets all three batteries paralleled. But, yes, the resistors are a bit cleaner and defined.

[RaymondR]

Stainless steel pushbike spokes make good high current resistors.

[jordanbigel]

I was thinking I wanted to use 10ft each leg - to get the resistance up to 0.1ohm (and thereby reduce the current to 30amps), but, then I multiplied by 6 wires instead of 4. Oops.

I think in the end you are right, 5ft would be fine because the current is going to tail off pretty quickly from what I understand, so after the first 30 minutes I expect the current will be well within spec for 10awg.

[goboatingnow]

You can balance them with much smaller currents , higher value resistance etc , all that happens is its takes longer for the voltages to equalise

You can also use any suitable dc load and individually discharge each battery to the same voltage . Monitor the battery final voltage with a good DMM.

The batteries don’t have to be all connected together to balance them.

[kirklkjb]

Originally Posted by goboatingnow

You can balance them with much smaller currents , higher value resistance etc , all that happens is its takes longer for the voltages to equalise

You can also use any suitable dc load and individually discharge each battery to the same voltage . Monitor the battery final voltage with a good DMM.

The batteries don’t have to be all connected together to balance them.

Since you are paralleling them you can just fully change each one as long as you use the proper settings for a single battery. Once fully charged by the same charger they should be at a very similar voltage. Check the final voltage with a DMM.

[Emouchet]

If you are afraid of draining too much current from the batteries during the equalization time, use thinner copper wire, like AWG 16 or 18. Should the current being too high, they will eventually operate as fuses should the BMS not fast enough to figure out too much current is flowing.