Batteries - Massive Parallel and Series Issue- 3 questions

Re: Batteries - Massive Parallel and Series Issue- 3 questions

First, make sure you have a "balanced" wiring for the battery bank (equal length current paths for all wiring paths). The Smart Gauge website has a good article.

Regarding understanding your battery bank, here are a couple of good Battery FAQs:

Battery FAQ
www.batteryfaq.org

Next, a good digital meter (suggest a DMM+DC Current Clamp Meter like this one for ~$60 is "good enough" for our needs here) and a hydrometer. The steps I would go through:

1. Measure and log the temperature corrected specific gravity for each cell. Never let plates be exposed, do not over fill cells, you should need to refill a cell about every 2 months or so--Filling too often may indicate over charging, going 6-12 months without filling may indicate under charging (different battery types can have different water requirements too).
2. Inspect wiring, clean connections and add anti-corrosion grease, re-torque bolts, etc.
3. Use the Digital meter to measure the voltage of each cell or each battery). You are looking for differences and absolute value
4. Use a DC Current Clamp meter to measure the current through each parallel string during both heavy charging/discharging--You want current balanced between all strings.

So, with all of the above information, you want:

• Battery to cycle between 50% and 90% state of charge during normal operation, never below 20% state of charge, and recharge >90% state of charge once or twice a week (at least). 100% state of charge (+equalization if needed) around once per month (this is subject to lots of debate here--seems to depend on brand/model of batteries--typically a bigger issue with larger industrial type batteries).
• The hydrometer will give you the "exact" state of charge for each cell. If there is a large variation between cells (on the order of 0.015 to 0.030 or or greater, try equalization to bring cells back together--If you still have very low cells, they may be sulphated and ready for replacement).
• The voltage measurements will show you high voltage/low voltage cells... May indicate open or shorted cells. Open cell can drop a string out of the bank, and a shorted cell can discharge the whole bank (while overcharging the one string it is in).
• Current Clamp measurements will show how effective each string is. If you see over a 2:1 between high and low string currents (i.e., 20 amps in one string,
• 10 amps in another), you probably have some issues to look at (bad electrical connections, open/shorted cell, etc.). Do the measurements both under heavy charging and under heavy loads.

A battery bank that is slowly going bad may indicate "deficit charging" (and low water usage would also help to confirm under charging)--Using more Amp*Hours from the battery bank than you are returning via charging. A larger solar array may be helpful here. Also, a Battery Monitor will make day to day operation much easier (and for spouse/kids/guests easier to understand how to take care of the battery bank without using a hydrometer or guessing with a volt meter).

If the battery bank is being heavily cycled, and you have sufficient solar resources, then you might try raising the Absorb Set Point (perhaps a 1/2 volt at a time) and/or increase the Absorb timer (from 2 hours to 4-6 hours), and/or reduce the Absorb termination current (if present in your charge controller).

Personally, I am not a fan of a massively paralleled battery bank. It makes maintenance more work (lots of cells/connections to inspect) and difficult to ensure proper current sharing between parallel strings. Also, each parallel battery connection should have its own fuse/breaker per string (for electrical safety)--And it gets expensive to do large numbers of circuit protection devices.

At 2,640 Amp*Hour battery bank, your "ideal charging current" is in the range of 132 amps to 264 amps to 343 amps (5/10/13% rate of charge). Those are pretty heavy currents and a very large battery bank. Do you have enough solar array and backup battery charging power (genset/mains/etc.) to keep them well charged?

My two cents worth of suggestions... I would look at a 48 volt battery bank (yea, new inverter, possibly new backup AC battery charger) to reduce charging current and the number of parallel battery strings. And I would look at larger AH rated batteries / cells. My personal suggestion is to aim for a 1 string battery bank with 2-3 strings maximum.

Questions?

-Bill

Re: Batteries - Massive Parallel and Series Issue- 3 questions

It can be a quick way to find hot spots when the battery bank is subjected to heavy current (look for over heated wires/connection points).

Overall, if the battery gets too hot (over ~110-120F), you would need to cut the charging voltage/current until the battery bank cools down.

I think the DC Current Clamp DMM + Hydrometers are your two most important tools here.

-Bill

Re: Batteries - Massive Parallel and Series Issue- 3 questions

I would term it more as attempt to avoid over 2-3 parallel strings--rather than say it won't work. Because, it does for many people (and industries).

Massively parallel banks just have a lot of maintenance/debugging/costs and other issues that make parallel strings a lot less desirable for somebody that does not want to add battery tech to their resume.

-Bill