LifePo4 Voltage drift shunt and balancer

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solgato asked Dec 30, '21

LifePo4 Voltage drift shunt and balancer

I’m experiencing voltage drift between my two 12V LifePo4 batteries wired in Series for 24V that I’m trying to address. When fully charged, there is a .20V deviation in resting voltage. This after charging each individually to within .01V to re-sync then letting them go through a daily solar charge cycle.

My questions are:

1. If my batteries are spaced apart (with all wire runs being of equal gauge and length) but with the shunt located nearer to one of the batteries, will that battery measure lower in voltage?

2. If I install a battery balancer, are there any special wiring considerations that need to be made with regard to the BMV and shunt?

3. I assume with the Battery Balancer it is also important to make sure all wiring is equal? Since my batteries are separated, the challenging part is tying into a mid-point between the batteries. The only real way I could do this is to run separate wires from each batteries terminal to the Balancer to supply the 0V terminal, and then wire the 24V and Common terminals to the positive/neg supply side of my DPDT Master Power switch. I realize the longer the wire runs the less accurate the Voltage value readings will be from the Balancer, but I think it will still work provided it sees more than 27.3V and measures enough deviation to activate.

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snoobler answered · Dec 30, 2021 at 03:32 AM

1. No. The batteries are a series circuit, so all current must pass through all components equally.

2. No. The amount of current that passes through the balance wires does not require large wire. Many balancers include their own wiring, and that is sufficient.

3. Not really. The balancer wires are best attached directly to each battery as applicable.

When in operation and discharging below ~90% SoC, how do the 12V compare?

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kevgermany ♦♦ commented · Dec 30, 2021 at 10:10 AM

There is specific information on how to size balancer cables in the manual. This must be followed.

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Answer 1 · 2021-12-31T05:21:40Z

solgato answered · Dec 31, 2021 at 05:21 AM

Well after reading the Victron balancer installation guide, the answer is Yes to my last two questions.

First, the unit does not have wire attached, therefore specific sizing is called out depending on system.

Also although not specifically stated, you can see in the diagram that the midpoint should be just that, with equal length wiring between batteries for an accurate deviation measurement, otherwise I’m pretty sure the deviation and voltage values (if wires are unequal in diameter or length) that the balancer sees may not be accurate.

As to my first question, I was asking about voltage drop due to resistance by having the shunt mounted closely to one battery that is in series with another that is located many feet away.

To test this, I removed the battery leads and measured both batteries and found they had equal resting voltages of 13.36V when disconnected, so the issue seemed to be related to the shunt. Read below.

With regard to discharging under load, that isn’t my focus. My focus is on the charging cycle and making sure each battery is reaching capacity with little voltage deviation at rest.

Something was causing one of the batteries to charge slower, and since I don’t charge to 100% capacity, the chargers were shutting off early and the batteries were getting out of sync with one reaching a higher V.

Upon further investigation, I found a poor contact issue on one of the shunts main Neg terminals. I cleaned it up (a little solder slag causing the lug not to sit flat) and reconnected everything, and both batteries V measured equal at 13.5V.

Hopefully this was the issue because the other problem I’ve been having is a random disconnect under load which I always suspected as being an intermittent issue and not any limitations or protections built into my BMS’s.

Unfortunately my shunt lives in a tropical marine environment, and DC current, salt, dissimilar metals, and humidity don’t mix well, so I’m not surprised to have found a poor connection.

Also my boat vibrates and gets tossed around a lot, and I think the soft solder slag had ground down a bit from vibration as the terminal bolt was not as tight as it should have been, making the connection even more suspicious.

Tomorrow I will measure each battery after the daily solar charge cycle, and next boating trip out I will see if I can induce a disconnect while the solar chargers are switched off.

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kevgermany ♦♦ commented · Dec 31, 2021 at 06:59 AM

Good that you found your problem.

I looked at this in depth for my system. Had quite a few issues but was helped a lot by @Alexandra (big thanks). Agree with your points, with a small addition.

The balancer will work well if wires are properly sized and close to equal in length and guage. It's important to realise that volt differences seen by the balancer are the product of resistance and current. So you need the same resistance between the three measurement points and the balancer. A poor fuse in one leg will skew things, as will different wire sizes, lengths and bad connections.

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solgato kevgermany ♦♦ commented · Jan 03, 2022 at 01:04 AM

Thanks. Yes, what you pointed out is exactly what I had figured. When you’re talking about hitting a specific voltage (27.3V) to activate the balancer, and then registering a midPoint deviation as little as 50mv to a start the balancing process, resistance with regard to wire gauge and length, and equal lengths between each battery are very important in order for the balancer to work as it is designed.

I may still have a problem, and suspect it might be the shunt itself, however one big gripe I have that wasn’t helping the situation is with regard to charging voltage.

Most people suggest not charging to max capacity in order to prolong life of lifepo4, and the problem with that (especially if two batteries are not in balance) is that the BMS doesn’t receive a high enough charge current for long enough to top charge balance the cells internally.

Furthermore, this philosophy of charging around 14.4V or a little less should be suggested more so if you are using your batteries more as a backup.

If however they are seeing some discharge daily and being charged by MPPT and not a standard charger, I think charging closer to 14.6V combined with a balancer is probably the best way to ensure your batteries stay in sync and reach 100% capacity.

In my case, my boat sits in my yard all day when not used and wakes up every morning for a solar charge cycle. During the evening when the MPPT’s shut off, the BMV and a wireless remote controller module draw about .13A which brings the batteries down a bit, just enough have them go through a bulk and absorption cycle each day, then they float the rest of the day at just over 27V with solar covering the parasitic draw of the BMV.

What I wish Victron would do is allow for the programming of a calendar based top charge cycle to activate the BMS internal balancing, very similar to the Equalization phase for FLA.

That way users could charge their lifepo4 at a slightly lower target V and reduced capacity to increase lifespan, but then say one day a week, or month, or whatever, the chargers will perform a top charge at a higher V and hold it long enough for the BMS to perform internal balancing.

Hopefully that makes sense.

I’ve decided to just start charging at 14.6V/29.2V and float at just over 27V because I found at 14.4V/28.8V with the slight imbalance between my two 12V 100ah batteries in series, one wasn’t seeing a high enough V to properly activate that batteries internal BMS, further causing one to drift out of balance of the other.

Obviously a balancer will help keep the two batteries balanced, but the batteries really need to be internally balanced as well, and the only way they’ll do that is if they hit a specific target voltage for long enough to sort out the imbalance.

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kevgermany ♦♦ solgato commented · Jan 03, 2022 at 09:22 AM

Good points. I guess that one thing victron are up against is the many different BMS systems on the Lithium side. There's no one size fits all solution. Just a lot of traps. And many battery makers seem to say charge to X volts, then stop, or have a short float.

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Martin Birrell solgato commented · Feb 02, 2022 at 01:06 AM

Yes good points. I have a 48 volt system with 4 x 12v LifePo4 batteries x 2 strings. I have the Victron SmartShunt and Balancers. Getting the system tuned with the correct charging has been trial and error having to deal with the 8 internal battery BMSs'. That said it has been well worth while as the maintenance of lead acid cells become painful over time.

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