Battery Loosing Charge

Re: Battery Loosing Charge

I am not an expert in batteries--but not needing to add water (assuming you don't have those hydrolyzing caps with the catalyst) could be a warning sign of under charging.

And from what I understand, the sulfates start to harden as soon as 24 hour after discharge. And, once the sulfates have hardened--they will not reconvert back. Here is one description:

If the battery is left discharged too long, the lead sulphate will form large, hard crystals on the plates and will not be able to be forced back into the acid. These crystals are large enough to physically clog the pores in the plate surface. The whitish appearance of plates is this permanent PbSO4 which seals the plate surface off from the electrolyte, rendering it useless.

It appears(from reading around) that if your battery SG and/or resting voltage indicates 75% charged (after charging and equalization), then it is probably sulfated.

Probably about the limits of my battery chemistry knowledge...

-Bill

Re: Battery Loosing Charge

Any of those C/xx values are perfectly valid numbers--just depends on the usage.

I sort of tend towards the C/20 values because--If we a assume a battery charges over a 3 day period (any longer, sulphation becomes a problem as it hardens, and worsens--apparently--at less than 80% state of charge), if you assume that you have 4-6 hours per day of effective sun (and/or your generator runtime per day), that works out to ~12-18 hours to charge... And if you assume the battery is used (heavily) over an 8 hour period per night, that again works out to a 24 hour rate over three days--not a 3x24=72 hour rate...

Technically, since a standard flooded cell battery is sized to 50% discharge level, those C/XX values should be 2x as large, and therefore somewhat more effective capacity--but I use the C/20 rate as one of the "hidden" safety factors to slightly error towards recommending a larger battery bank (allows for aging like sulphation, batteries operating cooler than ~77F, the occasional heavy loads like well pumps, etc.).

I would agree with Jim that discharging a battery using a heavy load would probably be more current than the C/10 or C/20 rates--but closer to a C/4-C/1 rate (assuming it does not damage the battery--and that I would this would be for a short time--10-20 minutes maximum or less required to measure an elevated temperature). At a C/20 or lower rate, it would be much more difficult to identify a "hot" cell as thermal transfer would probably equalized the temperature over the entire battery.

If you have two or more batteries (or cells where you can measure individual voltages) in series, using a (calibrated or known good) meter to monitor individual cell/unit battery voltages would help you identify a single unit for replacement in a bank (examples, 2x12v batteries or 3x6v batteries in a 24 VDC bank).

And this is another reason I like a series bank--it is easy to identify a weak/failing replaceable unit. It is much more difficult in, for example, a bank of two or more 12 VDC batteries in parallel in a 12 volt bank. They all have the same temperature, and a "weak battery" will simple supply less current--making looking for the "hot battery" possibly point to the only one actually supplying useful current.

For parallel banks, you would need to use other methods to detect weak units. Using a DC current clamp meter to measure individual currents, disconnecting the banks and testing each battery separately, monitoring specific gravity of all cells during test (gel and AGM types you can't).

I "trick" I have used to measure DC currents in parallel paths and I don't have a DC current clamp is I look for equal sized wire runs between the parallel circuits and place a DVM across one in each parallel path just like it was a shunt--works pretty well to find those connections that are carrying more or less than their fair value of current.

In the end, finding a "bad cell" in a battery can help with warranty claims (see, the rest of the battery is good--just one cell had early life failure). If you return an entire bank of batteries, all failing capacity testing, then it looks (to me) more like the entire bank was abused (poor charging, excessive discharging, sitting discharged for long time periods, etc.).

In the end, unless you do capacity testing when the bank is first installed and write the values and test conditions down, you really don't know if your batteries are aging poorly or if you got a bad set in the first place.

Lastly, you have been focusing on battery capacity issues... Another thing to check for are poor electrical connections (loose or corroded terminals, corrosion in cables, failed cable crimps, etc.). Using your tools to check for excessive voltage drops or poor current sharing, and if you have "heavy" loads, you can even check for hot spots in your wiring.

Bad connections can mimic flaky battery problems too... And they can be anywhere in the circuit (at the battery banks, bus bars, circuit breakers, inverter/charger connections, etc.).

-Bill

PS: I should add, to be clear, that the C/XX capacity is based on actual current draw over time, not average current draw over time (C/10 over 10 hours is, effectively, the same load as a C/10 load at 10% duty cycle over 100 hours). Because the discharge curves at different load ratings are not linear (a C/10 at 10% duty cycle rating will be lower than a C/100 discharge).

In my every humble opinion, a C/120 load for a Lead Acid battery would be a useful rating where you had a worst case steady state load of (say a cell phone tower installation) with relatively constant load over a three day period of no-sun and a discharge maximum of 50% (3x24/50% capacity=144 hour discharge rate). After three days, you would want to fire up the generator and quickly recharge the battery bank to prevent the sulfates from hardening.

But in the case of most off-grid solar installations, most loads are going to be in the evenings and mornings (assume 4 hours each), so the effective current the battery supplies is 8 hours per 24 hour day for thee days--using the C/20 (really C/24) rate capacity for planning.

In the end, there are so many derating factors (temperature, losses due to battery chemistry, losses due to equalization, manufacturing variations, aging, etc.) that you can go nuts trying to figure out the optimum capacity...

To simplify, we tend towards rules of thumb that get us "close enough" (50% maximum discharge, 3 days of no sun, insulated or controlled temperature environments, rounding capacity requirements "up", etc.).

On of my simplification rules from engineering is the rule of 2 and 10... If one thing is 10x (or more) larger than something else, then the "smaller thing" can be ignored. If something has 2x or less difference, then they are close enough to being equal in effect.

This, at first, does not seem like a realistic set of rules for life. For example, a 6 foot person vs a 12 foot person seems ridiculous to call "almost the same"... However, measuring height is a "linear measurement" of the differences. However, if we did this comparison using volume (or weight in this analogy), a person 2x as "large" would weigh ~8x as much (think 2*2*2 or the volume of a cube). In real life, we assume that the adult population's average weight is around 100-200 lbs--a real world example of a 2:1 range applied to humans... Most of the "calculations" in engineering are frequently based on area and volumes, not linear--hence the engineering rule of thumb of 2:1 and 10:1...

So, in my thinking C/10 and C/20 rates are pretty close to the same--so I don't get too excited about the difference between them. However, a C/10 vs C/120 rate is quite a difference and would demand close examination as to how it would affect my plans.

Another helpful rule of 10 is the 10C (Celsius) rule for temperature effects... The physics of "activation energy" for most common materials, once you do the math (details long forgotten from my schooling), works out to a change in temperature by 10C will result in a factor of two difference in effective life. For example, if you can cool your electronics by 10C, they will last twice as long. If you run something 40C hotter, its life will be 1/(2*2*2*2)=1/16 the life.

On the other hand, if you run your batteries 10C cooler, their self discharge rates will effectively halve (and their lifetime would double)... However, because of the chemistry, their capacity also drops (in this case, by roughly 10%).

Re: Battery Loosing Charge

If the tops of the plates were dry--then just cross your fingers and hope that all is OK.

Regarding rain water--I kind of doubt that is "clean" enough for battery usage. Distilled or Deionized water is pretty much the only liquid that should be used.

There are various home made and other types of stills that you can use at your cabin (solar or flame powered) if distilled water is not handy.

At this point, there is nothing more (that I am aware of) short of emptying your batteries and refilling them with new electrolyte (and even that is an iffy situation since you may not know what the current level of charge it)

-Bill