48v battery advice

Re: 48v battery advice

Thommo wrote: »

Hey Vic, Thanks for posting your comments - I think fortunately that I have come across this problem at an early stage. Yesterday I got a Hydrometer and checked all the cells as soon as I got home and with the exception of the one cell that I think (or more correctly... I know) is toast, all the rest show an SG of between 1.24 and 1.28.
First thing in the morning I check the volts for all the cells and they show about 2.08vpc - the dud one shows about -0.48v.

You need to monitor the voltage (and specific gravity) per cell over time and under various conditions (under charge, load, and resting after 3 hours or more)... The only "accurate" method is to measure the voltage after 3+ hours of no charging/discharging current to estimate the state of charge (and ~25C/77F is the cell reference temperature).

By my reckoning 24 x 2.08 = 49.92 - so when I get the new cell all aught to be well.......maybe....

In the end, you want to get the cells to at least 90% State of Charge several times a week (minimum). You are not after 100% SOC every day--that would be a waste of generator fuel and a bit hard on the batteries.

I called the guy who originally supplied and installed my system, but he's unfortunately living in Galicia in the north of Spain now (we live in Almeria in the south) and he's also not in that business now - so no help there. He gave us the name of a guy that he used to get his battery cells from - we phoned him and he said he could get us a replacement 2v battery cell, model 7OPzS490, but not made by Sunlight (the original Greek manufacturer).
Is this model number 7OPzS490 some generic type - I feel rather skeptical about having a cell from some other company. What do you think about that? - Do you think that a cell made by a different manufacturer would be ok as long as the tech specs were the same?

You really have no other option (unless you can find a used cell in good condition).

If must also ask your advice on the rating of the alternator in the genset. When I read the manual supplied by Xantrex it clearly stated that in the settings for the Amps in the AC2 input (the genset input ) should be set at the max output (in amps) of the alternator on my genset (which is 26a). I went to the menu to alter that setting, if need be, and found that it had been previously set at 15a.
I reset the value to 25a and the system does seem to be functioning a little better for that change (battery problems notwithstanding).

You may have to back the maximum current down a few amps--Depends on how well your genset tolerates the maximum current (some are better than others).

I know that there is a difference between the amps (DC required by the battery and the AC amps supplied by the generator but isnt there some credo that says that the charge (or charger) should be rated at 10% (c/10) of the battery A/H capacity - that being so shouldnt I need around 50a for the battery charging? (whereas I only have 25a - to cope with both the load and the battery charging)

We use a 5% to 13% rule of thumb as a starting point. Read the manuals for your batteries and make sure you are within their recommendations too.

Below 5% gets questionable as batteries are cycled (and get older--when they have higher self discharge). If you charge only during the day and discharge only at night, 5% can work... If you use power during the day, then you are removing charging current available to the battery bank--So you have to watch your battery state of charge (and water usage) to make sure it is getting recharged properly.

Over 13% rate of charge--Batteries can over heat (probably more of an issue for fork lift operations where batteries are deeply cycled every day). Also, it becomes a bit of a "waste" of money as for the typical users, the battery banks reach full charge very quickly and the excess generator (or solar array) capacity is "unused" much of the time. 10% to 13% is a good "balance" for most people (we don't like to see very large battery banks relative to loads--that drives larger solar arrays and gensets--And too large of gensets can waste fuel much of the time when running at less than 50% of output capacity).

I know that you'll set me strait on that - sorry - I don't have the math for that!

So, how much power do you use every day (AH @ XX battery voltage; Watt*Hours or kWH per day, any peak loads like a well pump, etc.).

What is the kW rating of your genset (I assume at 230 VAC and 50 Hz)?

If you have a large enough genset, it would not hurt to get a second AC to DC battery charger (~25-35 amps) and connect that up. You can parallel battery chargers to the same battery bank (with proper wiring). And the extra AC battery charger should not be too expensive (relative to your battery bank costs).

Hopefully, within a week we will have bought another cell and it will have been installed and at that time I'll do an 'Equalize Charge' to synch up all the battery cells.

Yea--You need to do this--And it may take a lot of generator run time/fuel to get things working well again... More or less (another rule of thumb) you want to equalize if the cells are more than 0.015 to 0.030 SG difference between the high and low cells. Measure the SG of each cell every 30-60 minutes--and when all cells stop rising, stop the equalization charging. Log the temperature corrected SG for each cell--and that is your "new" 100% SOC for each cell.

Also, monitor the battery temperatures too... You don't want to exceed ~120F (~49C)--High temperatures are very hard on battery banks too (more or less, for every 10C rise over 25C, your battery ages 2x faster--If 20C rise, battery ages 4x faster, etc... Conversely, for every 10C drop, the battery will age 1/2 as fast).

Do you have any recommendations at to what the max voltage the battery should be charged at for this 'Equalization Charge'?

Does your battery mfg. have a recommendation? In general, you want the the maximum equalizing current in the 2.5 to 5% range (500 AH, 12.5 to 25 amp range). That may take 60 to 63 volts or so (many battery chargers may have difficulties reaching those high of voltages). Monitor the current flow--As batteries "get hot", their required charging voltage does fall (so you could start at 62 volts and need to fall back to ~60 volts as the bank heats up).

Equalization is "hard" on a battery bank, so you only want to do the minimum amount required. Then stop. During equalization, charging current is converted into hydrogen/oxygen gases and heat... So, they tend to get hotter faster at lower charging currents.

Also, monitor electrolyte levels... Never have exposed plates. But don't fill too high either--As gas forms inside and batteries heat up, the electrolyte levels will rise and can flood over the top of the battery pretty easily (fill low cells to only 1/3 or 1/2 levels when cold/before equalization--Add water at the end of the equalization charge--and again don't over fill).

Regarding water usage--If you have to add water every two months--You probably are OK. If you are hardly ever adding water (more than 6 months between fills), then you may be under charging (there are some low water usage batteries that use less water than "normal"). If you are adding water every month or every few weeks, then you may be over charging.

I have a feeling that the current value for this setting is a tad low - 57v or thereabouts.

Make sure your volt meter is accurate and you are measuring the voltages at the battery (not at the battery charger--wiring voltage drop can give you inaccurate readings during charging).

And, you probably would be better at 58-59 volts. During winter you may need to keep the voltages a bit higher to make use of all of your available sun... During summer (and when you are away on vacation) you can probably dial the charging voltage down a bit (temperature compensated charge controllers will do some of this automatically anyway as the battery banks get hot/cold during the seasons).

Good luck Peter.

-Bill

Re: 48v battery advice

Greece is having terrible economic/political problems right now (as I am sure you know)... I did not see any news about Sunlight SA. So don't know if related or not (company appears to still be operating in Greece).

I wonder if you could take the cell to a local battery rebuilding shop and see what they think.

If the cell was filled with water--I am not sure if the battery bank would even function (battery should have fairly high resistance). You should have noticed the cell getting hot during charging and a lot of gassing (I would guess). If there was a lot of gassing, then the cell may have been damaged by so much oxygen generated during current flow (again, guessing)...

I would suggest recharging the cell until the SG does not rise any more, drain the battery and refile with new electrolyte and see what happens... However, since I know nothing about batteries at this level--My suggestion could be quite dangerous--Or just a waste of money for the acid on a cell that should be scrapped...

-Bill