48 volt bank help

[Jemplayer]

Hi all,

New here. Been living with an off grid solar system for a year now. As I get more familiar with it all, I start drilling down in areas that I want to better understand.

One of these is series wired battery banks. I have a set of 8 - 6volt us battery 232 ah batts in series making 48 volt, and 11,6 kWh . They are connected correctly as in positive one end and negative the other. They are also connected with 35mm wire between the batts.

The issue I find is that the end two connect batts (we're the bank is connected to the inverter and mppt), are more charged than the rest in SG reading. It seem the next two in line are also a touch higher in SG readings, is this normal. Is there any way to try distribute charge and discharge better.

I though of using another 35mm cable to jump positive and negative from each end into the middle of the bank thus trying to distribute the current going in and out more evenly.

Am I way off of here? Or does this hold any promise?

Thanks for any input here,

[paulcheung]

You need to Equalize your battery bank.

[mapmaker]

The issue I find is that the end two connect batts (we're the bank is connected to the inverter and mppt), are more charged than the rest in SG reading. It seem the next two in line are also a touch higher in SG readings, is this normal. Is there any way to try distribute charge and discharge better.

Welcome to the forum,

You are extremely wise to be checking up on your batteries with an hydrometer. All battery cells are a bit different and there are two common ways of dealing with that... for lead acid batteries we do it by a controlled overcharge (equalization). Flooded lead acid batteries are relatively tolerant of overcharging. It is dangerous to overcharge some types of batteries (lithium, for example) and a battery management system is used to keep them equal.

The pattern of SG you are observing in your battery bank is probably related to uneven temperature in your battery box. There a few ways to deal with that... depends on exactly what's going on to cause the temp difference. If you can't correct it, perhaps you should rotate the batteries into different positions in the box.

I though of using another 35mm cable to jump positive and negative from each end into the middle of the bank thus trying to distribute the current going in and out more evenly.

Bad idea. Sparks will fly and batteries will explode!

--mapmaker

[Jemplayer]

Welcome to the forum,

You are extremely wise to be checking up on your batteries with an hydrometer. All battery cells are a bit different and there are two common ways of dealing with that... for lead acid batteries we do it by a controlled overcharge (equalization). Flooded lead acid batteries are relatively tolerant of overcharging. It is dangerous to overcharge some types of batteries (lithium, for example) and a battery management system is used to keep them equal.

The pattern of SG you are observing in your battery bank is probably related to uneven temperature in your battery box. There a few ways to deal with that... depends on exactly what's going on to cause the temp difference. If you can't correct it, perhaps you should rotate the batteries into different positions in the box.



Bad idea. Sparks will fly and batteries will explode!

--mapmaker

Thank for the info, and the warm welcome. I spoke with the guys at US battery. The first and last battery in a string are prone to a higher SG level. I get 1270- 128O on most cells. The last batt in the chain i.e. the negative connection, is the has highest SG at around 1290 due to it being last in the string.

I am going to play around with the charge voltage to get the best balance of fully charged cells without causing the last batt SG to be to high.

[mapmaker]

I spoke with the guys at US battery. The first and last battery in a string are prone to a higher SG level. I get 1270- 128O on most cells. The last batt in the chain i.e. the negative connection, is the has highest SG at around 1290 due to it being last in the string.

I think that is nonsense. In a series string the current is the same in all batteries. The voltage across each battery is determined by the internal resistance of that battery.

In theory, if there was a high frequency component to the charging current, it might affect batteries differently due to capacitance, but I have never heard of such a effect in real life battery banks.

As I mentioned previously, I suspect that if what you're seeing is not just a random variation in your batteries, it is because of a temperature differential.

--mapmaker

[Jemplayer]

In addition, I find that after watering my batteries, even a small amount like 90ml per cell, instantly reduces the SG level to around 1240 (they were 1265-1270 odd before) and then seems to take me having to EQ for a couple of hours to get the level back to spec.

Does anyone else always EQ after adding water, even a small amount. Or does your batt just normalize after a couple of Charge/discharge cycles. As I am not finding that..

[Dave3011]

In addition, I find that after watering my batteries, even a small amount like 90ml per cell, instantly reduces the SG level to around 1240 (they were 1265-1270 odd before) and then seems to take me having to EQ for a couple of hours to get the level back to spec.

Does anyone else always EQ after adding water, even a small amount. Or does your batt just normalize after a couple of Charge/discharge cycles. As I am not finding that..

Sorry I cant talk from experience as I'm from the SLA camp. But it would be quite obvious to me that adding water would reduce the SG. Logic also tells me that if your batteries have been losing water by evaporation then the SG should go up. But I dont know I'm not a chemistry fanatic when it comes to batteries. Would be interesting to hear from someone that knows though!

[mapmaker]

Does anyone else always EQ after adding water, even a small amount.

I do. But I don't add the water until the battery reaches float after a full absorb.

But it would be quite obvious to me that adding water would reduce the SG.

The logic is correct, but the drop in SG that you measure is disproportionate to the amount of water you add. This is because of stratification... the fresh water 'floats' on top the heavier electrolyte.

Logic also tells me that if your batteries have been losing water by evaporation then the SG should go up.

Depending on several factors, primarily temperature, most of the water is lost by the electrolysis (not evaporation) of water into hydrogen and oxygen.

--mapmaker

[Sunking]

In addition, I find that after watering my batteries, even a small amount like 90ml per cell, instantly reduces the SG level to around 1240 (they were 1265-1270 odd before) and then seems to take me having to EQ for a couple of hours to get the level back to spec.

Well the SG is suppose to drop when you add water, but you do not EQ after adding water. If your batteries are low on water, your SG should be higher than normal assuming they are fully charged up. When you add water, th eSG should drop to normal. Unless of course you have over charged your batteries and electrolyte has boiled out with water. If that is the case you will never reach the required SG, and would have to add acid. Bu tunless you know exactly what you are doing, never attempt to add acid to your batteries. One ignorant mistake can leave you blind, severely burned, or dead from toxic gasses.