March Solar – Battery Advice
Batteries are mystical magic boxes in which we store electrical energy by
chemical changes. Much of today's battery technology is a slight refinement
of what we knew 45 years ago. If you know of a more economically viable way
to store electrical energy, tell us! It will be the next wheel !
Several warnings are included in this document please read all of them; the
most important being:
WARNING:
This may surprise you but those innocent looking batteries can be extremely dangerous, your battery room should not be accessible to people unfamiliar with the risks of batteries; they can kill you if you don't treat them with the caution that is required. (use your legs...if you don't pick them up properly, there goes your back) When working with batteries use extreme care, insulated tools, protective gloves, footwear, and goggles. Remember: a typical P.V.P.S. battery bank can produce 6000 amperes if shorted. (that's enough to weld tools to terminals, cause burns, or STOP YOUR HEART DEAD.)
DISCLAIMER:
All of these statements are made from our experience using mainly large deep
cycle wet lead acid batteries in conjunction with Photovoltaic Power Systems.
They are opinions, if you have knowledge you wish to share with me... please
do so.
I am not a chemist, however with that said I can tell you there are very
few people who really understand these wonderful "Black Boxes".
If you have recently purchased a Photovoltaic Power System (P.V.P.S) you know how expensive batteries can be! Batteries are our industries achille's tendon but until somebody comes up with something better here are some tips that should help you realize more years of use from them. If you pay a bit of attention to them it will pay you in the long run. (just like everything in PV!)
ALL batteries require a bit of maintenance (even sealed units should have their connections checked)
LOCATION:
The location of your battery bank is important; keep in mind that you want
everything close together to reduce system losses but you want to isolate
your batteries (i.e.: a separate container or room that is vented to the
outside) The venting is important to avoid a buildup of hydrogen gas which
can occur during charging and is explosive if ignited. (Big Sign: No Smoking
near batteries)
For these reasons (and the fact that battery acid is corrosive) you want
to locate all of your electronics in a room other than the battery room.
A spark that you can't see from an inverter is enough to ignite hydrogen
gas. Batteries should be on a rack or support that allows free air movement
all around, above and below the batteries. Never put batteries on a floor!!
So Remember: Ventilate and separate!
General Battery Stuff
Discharge rate:
Generally, if you discharge a battery at a slower rate its capacity is
slightly higher than had you discharged it quickly.
Temperature:
Lower temperatures affect batteries by lowering their amp hour capacity to
a great degree. Very high temperatures will promote water loss and reduce
the number of cycles your batteries are good for. Therefore its best to keep
batteries as close to room temperature as possible and failing that try to
reduce the possibility of severe or rapid temperature swings.
Freezing:
Yes a battery can freeze because part of the electrolyte is water; but a
maintained battery will not freeze under normal operating conditions. i.e.:
an average deep cycle lead acid battery at a 50% state of charge will freeze
at about -24 degrees centigrade and -16 when 75% discharged (which will never
happen ...right? ) However, even with this in mind we prefer clients to bury
insulated battery enclosures to help protect batteries from extreme temperatures.
Depth of Discharge:
All batteries are affected by depth of discharge; the less you take from
a battery between recharging the more cycles you will get from it (i.e.:
if you deep discharge your batteries frequently they will not last very long)
Batteries will self discharge when not being used; generally with lead acid
cells you can guess it to be around 5% per month. (however it does increase
with higher temperatures) A battery that is only allowed to be 30% D.O.D.
(depth of discharge) could last three times the number of cycles as one that
is 50% D.O.D.
March Solar's Recommended General Battery Maintenance
When working in the battery room I like a nice rubber pad to be on (it's more comfortable and isolates me a bit from ground) Even an old piece of thick carpeting can be of help.
I also like a big bucket of Baking Soda and some water nearby !!
(They are quite effective in neutralizing small amounts of spilled acid)
- Check total battery bank voltage (if using a separate meter make sure it is within 5% of your control panel meter)
- Check individual cell voltages; look for any that are out of sync with the others. Try to check everything in the same order every time and take notes. (Ask March Solar for our battery maintenance sheet)
- Check that all electrical connections are tight and clean; watch for corrosion.
- Check battery tops; they should be clean dry and secure.
- Check your battery bank enclosure, make sure your rack is in good shape and nothing can fall onto or interfere with the batteries, make sure your enclosure is securable.
- Check your electrolyte level in each cell of each battery and ADD DISTILLED WATER IF NECESSARY: never allow electrolyte level to fall enough to expose plates to air.
- Always record any actions taken.
Checking State of Charge with a Hydrometer
To measure specific gravity use a hydrometer.
(Careful... you're playing with acid !!)
To use Hydrometer:
- Squeeze Bulb
-
Lower into electrolyte and slowly draw electrolyte into hydrometer
2a) on the first cell empty the electrolyte back into the cell and repeat step 2 four times to bring the hydrometer to the same temperature as the battery - Take the reading carefully. The sample must be large enough to float the inside tube, keep it straight up and down when taking a reading.(be certain the float is free and unimpeded)
- Carefully return electrolyte to same cell
- Check specific gravity against chart for state of charge.
- All cells should be close to one another.
Specific Gravity State-of -Charge Chart
| SPECIFIC GRAVITY VALUE | OPEN CIRCUIT VOLTAGE | STATE OF CHARGE | FREEZE POINT (Celsius) |
| 1.265 | 12.60 | 100 % | |
| 1.245 | 12.57 | 90 % | |
| 1.230 | 12.54 | 80 % | -32 |
| 1.216 | 12.45 | 70 % | |
| 1.202 | 12.36 | 60 % | |
| 1.190 | 12.27 | 50 % | -24 |
| 1.175 | 12.18 | 40 % | |
| 1.160 | 12.09 | 30 % | -16 |
| 1.148 | 12.00 | 20 % | |
| 1.120 | 11.70 | DEAD | - 8 |
Typically, you do not want to go more than a 50 % D.O.D.
(depth of discharge) if you want your battery bank to last.
March Solar's Recommended First Aid Procedures
When dealing with your battery bank be prepared for the accident that will never happen!
-
If battery acid gets in your eyes:
flush with water for 15 minutes (to draw out the acid) and seek immediate medical attention. -
If acid gets on your skin (even with those gloves on):
pour baking soda on it (which you have handy) and then flush with water for 15 minutes (to draw out the acid). Seek immediate medical attention. -
The unthinkable: if someone has managed to ingest acid:
Rinse mouth and drink a glass of water (to dilute the acid). Call poison control.
Seek immediate medical attention.