For battery health/capacity we bought a pretty cheap static 50A battery tester from www.harborfreight.com ($10, item 93784). The 50A is close to the 3C/54A load (3 times “Capacity” which is 18Ah=54A) recommended for static testing batteries. We added a set of anderson connectors so we can simply plug in the battery and read the voltage which provides an indication of SoC. A flick of a switch provides a short 15 second test to determine health/capacity.
Bud and 1073,
I like your pamphlet, very professional looking. I would like to add one thing to your list of "Don’t"s. Don’t hold or lift the battery by the wires. The terminals are kind of fragile and will break just inside the case where they are not repairable. That and it’s really dangerous when dropped. Batteries should be handled by two hands.
I am working on a theory that you might be able to help with. I saw a lot of teams with vastly reduced life on their batteries last year. I am thinking that high discharge currents caused the internal plates to warp and produce a spot where the glass mat was compressed. I need more info so if you see newer batteries with reduced capacity can you question the team on how much current their robot draws in operation? i.e. turns, multiple functions, how many matches can they run on one battery, etc. I am particularly interested in drive train types vs. battery life.
Thanks, nice work!
Oh yeah, seen that one too many times. Probably been guilty of it myself on occasion. The problem I’ve seen in lifting by the wires is it loosens the bolts at the battery connector. This increases the resistance at that joint which reduces the voltage in the rest of the circuit under load. I’ve wanted to add some velcro (soft material) patches to the batteries to allow something like tubular nylon straps and the hook material to be used to help extricate the battery out of the robot. The team tends to make them a tight fit which in turn makes it difficult to extract them out for changing because of friction fit. This is the no. 1 reason I’ve seen students use the wires - to help get the battery out of its holder.
As an aside, I’ve picked up some temperature strips and slapped them on the batteries and motors - I’m wondering what the temperature is like after discharge/charge cycles. I know it takes a while for the temperature to migrate to the case which is why I what to measure it with the liquid chem strip – might be interesting data. I’ve got a couple to give away at the BAE NH Regional (like 5-6). Anyone reading this, track me down in the 1073 pit area and ask for one. They aren’t doing me any good sitting in my coat pocket.
I’m not convinced that high current loads warp the plates, that would require differential heating which I suspect would be difficult to achieve with the lead plates in the AGM technology. Not impossible, but not sure it happens. I did manage to warp plates on a fork lift truck flooded cell battery once - the battery was 3’x3’x4’ and I accidently dropped a screw driver shaft across the terminals. Vaporized the screwdriver and popped all the vent plugs off. Exciting.
The depth of discharge (DoD), that is deep discharges of 75% DoD and how long the battery stays at anything below 75% SoC have a lot more effect on battery life. With high current discharge you can hit 80%-90% DoD in a matter of a few minutes. The life cycle charts for the battery lists the test conditions as using only ~0.2CA (~5A) discharge over 3hr. My random guess is that you’d have to derate the lifetime cycles in the specification chart if you used higher, like 3CA (54A), discharge rates. Setting up and measuring the internal resistance seems like something definitive that could be checked on batteries exhibiting early capacity issues as that is a good indication of how “old” a battery may act.
OK Bud,
For the loosened terminals, I use and suggest a simple star lock washer(or internal or external lock washer) placed between the battery terminal and the wire terminal. I also use a star washer under the nut and screw head when screwing everthing together. The reduced surface area is more than made up for as the washer bites through surface oxidation, distributes the load on both terminals for a better connection and locks the joint together so it can’t be turned.
As to the plates getting hot spots, I recently read an article that focused on post mortem battery failure of the AGM type battery. A few of it’s advantages are thinner lead plates since the electrolyte is held tightly against the plate by the mat and less electrolyte due to the distribution of electrolyte ions being in such close proximity to the plates. All of this leads to a slightly lighter battery. Standard gell cell designs of a few years ago, in this battery size, were a pound or two heavier. The post mortem showed minute plate deformations that in some cases actually penetrated the mat and touched the opposing plate. Although the plates in the remainder of the cell were fine, the individual cell showed a reduced capacityor failed entirely. We use the West Mountain Radio, CBAII analyzer which will allow a varied load up to about 7.5 amps and will graph and calaculate the amp hr capacity. On several batteries, including some directly from the retailer, we found reduced cell capacity in only one or two cells. At discharge periods based on terminal voltage, we have recorded 2 volt drops far before the specified amp hr rating of the battery had been achieved. I published some of these graphs during the end of the 2006 season here on CD. I will include them in my presentation at the Championship as well. Following last year’s season, other teams came forward with stories about reduced battery capacity in some of their batteries. I know this is far from a scientific investigation but it seems pretty widespread. I am just looking to gain a a running total of teams, drive trains and battery problems. Some teams have none, and we had only one infant death before last season.
As to the temperature strips, I think you will find that when properly applied, you will see significant case temperature rise, also based on particular drive train designs or multifunction robots. If you closely examine the case and the support structure on the top of the battery it is easy to see the individual cells. Mount the strip in the center of a cell rather than at the cell boundary and I think you will see the highest temperature rise. I have experienced elevated temps on several teams, a few to the point of case distension. It will be interesting to know what you find.
This post is 12 years old, so my guess is that we likely don’t have it floating around anymore, but I’ll ask one of our other mentors to see if we can get that for you!