Hello, people! I’m sorry if this is the wrong place to post this, but I figured it was relevant enough.
Another fellow mentor and I have been wondering how we should be charging our batteries so that they’ll last. I know batteries aren’t meant to last forever, but we’ve had batteries that die sooner than others.
Last season during a regional competition, I visited another team that we’re friendly with so that we could check and see if our battery was acting up, or if it was just the charger. Our chargers will say that a battery is half-charged on some batteries, but it’ll stay like that, never increasing. In this case it was the battery, and she recommended that we keep batteries on a low-amp charge over the period we aren’t using them actively. While that does make sense (keeps the chemicals active) is it practical? I know other kinds of batteries will over-charge and be damaged that way. So what do you guys think?
The single biggest killer of lead-acid batteries is leaving them sitting around uncharged. They sulfate and likely will lose a lot of their capacity.
One solution is to always leave the batteries on a trickle chargers, as was suggested to you. If you have a quality charger, it can handle trickle charging without overcharging and exploding your battery. You other option is to pull the batteries out once a month or so to charge them back up.
I think a good charger makes a world of difference.
We overcharged several batteries to death using the chargers that came in the Kit of Parts many years ago. The Schumacher 2/4/6 amp Speed Chargers provided in recent years are much better; they correctly drop to a “charge maintenance” mode when the battery is fully charged. I’ve not heard any real complaints about the 3-bank RS3 that AndyMark sells, except that they don’t have a state-of-charge “meter” and you can’t force them to a lower charge rate when you aren’t in a hurry.
The robot batteries, if fully charged when stored, can be left for several months without damage. They will need to be recharged prior to use as each month will drain a little charge. They should not be stored in temperature extremes, i.e. less than 32 or greater than 90 F for any length of time.
Robot batteries have a finite life of 400 charge discharge cycles. If your design drains a battery in one match consider that life to be reduced significantly. I would suspect that such a robot could easily reduce that life to 200 cycles or less.
Do not use a trickle charger and leave it connected for long periods of time. Trickle charger is just a fancy name for a low powered, manual charger that will overcharge the battery and significantly shorten it’s life if left connected continuously.
The best plan it to use a high quality charger/maintainer. They will charge at ~14.2v until it senses the battery is fully charged and then either switch off, monitor battery SOC and then switch back on as needed or switch to float mode of 13.6~13.8v which will prevent the battery from discharging w/o actually causing it to charge.
Other tips to maximize battery life.
- Recharge as soon as practical after discharging.
- Do not excessively discharge the battery, Discharging below an open circuit voltage of 11.0v shortens battery life significantly. Preferred is to not allow the open circuit voltage to drop below 12.0v. Note open circuit voltage is best measured after the battery has been left to rest for 24hrs without being subject to either a charge or discharge.
In the absence of a charger/maintainer, float charger or battery tender the best plan of action is to charge the battery, disconnect it and after it has sat for a month or so subject it to a bit of a draw and then recharge.
- Don’t store your batteries in a discharged state.
- If you can, discharge(from a fully charged state) each one of your batteries over a 20 watt load until they are at 10.2 volts. Charge the battery immediately afterward. This can be done with a DC lightbulb and a voltmeter.
- Doing this about once per season will keep your batteries lasting for years(my team still has a battery from 2009 that works for practice). Measuring how long this discharge takes will allow you to calculate the capacity of the battery in watt-hours. If the capacity dips below about 100 watt-hours, the battery probably won’t work well at competition.
- A watt-hour meter on the robot during a match or a simulation of a match(driving the robot around sporadically using its special systems for 2.5 mins) may give you an idea of how many matches a battery can be used in.
Discharging the battery until it reaches 10.2v permanently damages the battery and should not be done, anything below 10.5 (or 11.0v depending on battery mfg) is considered battery abuse. For max life do not discharge below an open circuit voltage of 12.0v aprox 50% SOC.
I believe he said 10.2v under 20 watt load, not open-circuit. Does that make a difference?
Yes it certainly does, but since you need to let the battery sit for 24hrs after the discharge to get a valid open circuit voltage, you must break the 1 rule of maximizing battery life, recharging as soon as practical after discharge. So even if the open circuit voltage rebounds to 11.0v you still have abused the battery. It is highly unlikely that there will be much rebound after a low load discharge like 20w. Typically a battery will only show significant rebound if it has be subject to a high discharge rate for a relatively short period of time.
cheese,
The batteries are specified in Amp Hours. To discharge 100 watt hours would be to drain at 8.3 amps for an hour or about 0.5 C. While this is not a harmful or beneficial discharge, it alone is not going to extend life.
The 10.2 volts represents the manufacturer’s test cell voltage of 1.7 volts/cell. This is the point at which the manufacturer tests the AH ratings at various discharge rates. It will not be damaged at this voltage loaded. It is at this point that the cell voltage rapidly starts to nose dive towards zero volts. (The corner on each of the discharge curves.) For the NP18-12, self discharge to 50% rated capacity is greater than 12 months at 20 degrees C, longer if it is colder (within reason).
I am not one to recommend open circuit voltage testing to determine battery condition. Open circuit does not measure increased battery impedance due to other factors. The Battery Beak or the CBA III both test at some load to determine battery internal impedance and therefore actual health. Our batteries should test at about 0.011 ohms. I have seen batteries that test fine open circuit only to later find reduced cell capacity due to broken plates internally. A fully discharged battery will after several minutes, actually measure 12 volts. These will even under load show fine for a while and then suddenly drop 2 volts when the defective cell cannot supply any current. Search for one of my posts from a few years back on this subject. I posted comparative curves for good batteries and defective ones.
Open circuit voltage testing does not test the “health” of the battery only the state of charge. It is not the best way to test the SOC, a hydrometer is the best way, however with a sealed AGM battery that is not possible, so open circuit voltage is the only way to test the SOC.
The Battery Beak will give insight as to the health of the battery by measuring its internal resistance in addition to giving the SOC.
Yes battery mfgs base the AH rating on discharging to a loaded voltage of 9.6~10.5v but they do not recommend discharging the battery that low on a regular basis IF you want to maximize battery life. Here is the spec sheet for the Enersys FRC legal battery, from team #955. Note on the first page the expected battery life based on cycles and on the second page the percent remaining capacity vs open circuit voltage graph. 1200 cycles for a Depth of discharge of 30%, open circuit voltage 12.2~12.5v vs 250 cycles with a depth of discharge of 100%, open circuit voltage 11.2~11.8v.
Mr. V,
After extensive testing (by MK and others) a few seasons back, the rate at which the average robot draws current puts the DOD at greater than 50%. The engineers at MK put our expected life cycle at about 400 cycles on average. For teams in general that will give a good three to four years of use. However, with teams that draw down the battery in one match (obvious cRio reset at the end of the match) that life expectancy is two years or less.
Al, My point exactly, FRC usage is not the best for the life of the legal batteries so why would you purposely shorten it more by subjecting it to a complete discharge as the one poster suggested?
Agreed.