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Unread 07-29-2015, 10:11 PM   #31
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Default Re: Passive Voltage Bypass for 8v battery

What would be the component values for a 6 x 6v pack? I'm not sure what the optimal voltage would be for the 6V, but the linked article said to buy 2 x 6.8V 5W zenners. I'm a little challenged in the electrical department on some things, so does anyone (Sergio or 55BigBlock?) have step by step instruction on how to build and install these in Parallel and how to connect them across the batteries? Would it just be Pos -> Neg on each battery?

Help would be appreciated. I'd like to get this 4 yr old pack in balance and prolong their life as long as possible.

Thanks.
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Unread 07-30-2015, 12:59 PM   #32
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Default Re: Passive Voltage Bypass for 8v battery

These devices really work better if you start with batteries that are not too out of balance as a preventive measure.

The 6v batteries have a pretty high AH, so depending on how bad out of balance the pack is, You may not be able to "by-pass" enough AH to compensate.

That being said, this is what I think would work best:

The 6.8v zenner in my opinion is too aggressive to be used after the pack is balanced or if You start with a balanced pack. It is however needed if the pack is already out of balance.

I would prefer a 7.5v zener for maintenance on a new pack.

So, the best option would be to build both versions and use a 6.8v in parallel with a 7.5v zenner on the batteries with the highest voltage.

Just to be clear, you cannot put zener diodes in parallel, I am referring to parallel assemblies of Zener+Resistor.

The idea is that the 6.8v assembly would start to by-pass current once the cells on the best batteries gets above 2.3v, and the 7.5v assembly would add a second current path once the cells get above 2.53v.

That will allow those batteries to fully reach equalization voltage.

You don't even have to worry about the low voltage batteries until they get close in voltage to the good ones, you can stack multiple assemblies just on the good batteries and re-distribute them later. Each assembly connects to the + and - posts of a battery.

I did a quick sketch on the layout, just use the steps from the first posts to visualize the actual components.

The 10w resistor is oversized simply to act as a heat-sink for the zener diode. You need to follow the steps and "JB-Quick Weld" the zener to the resistor to promote better heat transfer.

You may need longer black/red 18ga wire leads since the 6v batteries have the posts on different sides of the battery.

I just checked on mouser.com and they don't have the same resistor I used in stock, so the link is for another manufacturer but the same part.

You should buy 10 of each just to have spares and use the ones that measure the closest to each other (per first post).

1N5342BG - Zener Diode 6.8V 5W
$0.35 (qty:10)
http://www.mouser.com/ProductDetail/...2fmwM2luBWw%3d


1N5343BG - Zener Diodes 7.5V 5W
$0.35 (qty:10)
http://www.mouser.com/ProductDetail/...88Ww7TmXiMc%3d


TUW10J2R0E - Wirewound Resistors - Through Hole 10watt 2ohm 5% Axial
$0.64 (qty:10)
http://www.mouser.com/ProductDetail/...Ugxz%2frBuc%3d

Let us know how it works if you decide to build them.
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Unread 07-31-2015, 12:04 AM   #33
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Default

Thanks Sergio. I already had those resistors and 6.8 zeners in my mouser cart, waiting on a reply. :)

My voltages range from 6.31 to 6.42. So to clarify, I can build one assembly at 6.8, one at 7.5, and stack both of them simultaneously (literally one on top of the other) on one battery. So if I have two batteries that are 6.41 and 6.42., I can stack 2 assemblies (6.8 and 7.5) on each one, and leave the other 4 batteries that are 6.31, 6.33, 6.35, and 6.36 with no assemblies installed?

After several charge / discharge cycles that, at the cutoff, allow the current to bypass the good batteries and continue to provide a charge to the weaker ones, I should hopefully see the lower voltages coming up, right? That should continue to charge the weaker ones while not boiling out the stronger ones, I think. At which point do I start adding assemblies to the other batteries? Do I add resistors to the batteries that are at or above 6.36, ones that are within .1 or .2 of the highest? I'm not clear on that point.

I think I do understand that the 6.8 assemblies will be used to try to rebalance the pack, and add the 7.5s to strong batteries if I have two or three stragglers still trying to coming up to par. If I am able to get the pack, say, within .01 or .02 of each other, is that when I would move to 7.5s on all?

And now, I have to start researching equalization voltage. I've seen he term on here a few time, but don't really know what that means as a practical application.

Thanks,
Paul
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Unread 07-31-2015, 09:21 AM   #34
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Default Re: Passive Voltage Bypass for 8v battery

This is similar to what most BMS systems do for Lithium batteries. I have a processor managed system for balancing my LIPO batteries that I use for electric model airplanes. They have 3 cells and they last way longer if you keep them balanced. Battery voltages are temperature sensitive so a passive system like this will work best at nominal temps. A processor based system can be temperature compensated.
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Unread 07-31-2015, 01:23 PM   #35
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Default Re: Passive Voltage Bypass for 8v battery

Definitely no performance comparison to a real BMS, but for approximately $4 per battery it does give You a small benefit.

Since You are likely buying 10 resistors for the price break, I would go ahead and make 10 assemblies, (6) for 7.5v and (4) for 6.8v. You may want to use different color zip-ties to differentiate the voltages.

The 5/16 terminals for 18ga wire are really thin, so You can stack them from multiple assemblies on the same battery post without problem.

Just make sure the battery cable is the first one on the post so the lug makes contact with the lead surface of the battery post.

Given Your battery voltages, I would place them as follows:
6.42v battery = (2) 6.8v + (1) 7.5v
6.41v battery = (2) 6.8v + (1) 7.5v
6.36v battery = (2) 7.5v
6.35v battery = (2) 7.5v
6.33v battery = nothing
6.31v battery = nothing
If the 6.33v battery starts to catch up, You can move one of the 7.5v from the the ones with (2) 7.5v to it.

It takes a while for it to have noticeable effect, so You don't need to check it very often.

When You get a new pack, You can install the 7.5v assembly in each battery from the start.

Also, find out if Your charger has an "equalizing" set (or procedure), which basically raises the voltage to an equivalent of 2.5v per cell. If You do that once a month, it will help out the process.
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