Battery Balancing and Charging a comprehensive approach - Fairly old, and focused on lead-acid batteries
These are some ideas about how to best keep a lead-acid AGM battery pack balanced. This will appear in the V is for Voltage Community Library > EV Collaborative Hand Books > EV Batteries Collaborative Hand Books. Please feel free to add to or criticize my ideas, and I will update this post. I have done real-world testing to develop my opinions with many off the shelf components including PowerCheq units, Zivan charger, Soneil charger, and BattPro battery regulators.
The three main objectives are to never overcharge any of the batteries in the string or the entire string to prevent gassing & heat, charge each battery in the string to 100% SOC when charging is finished to prevent sulfation, and prevent cell reversal by keeping the individual battery voltages constant.
1. Preventing Overcharging
The charger needs temperature compensation. Charging voltage vs. temp chart. This is so that it will be switched to the proper charging stage at the proper time preventing heat buidup, and so that it will never go over the gasification voltage.
Individual 12v chargers are better than a high voltage charger in combination with powercheq units to prevent overcharging, but only if they have temperature compensation. I don't know of any with temperature compensation, while a few high voltage chargers do such as Zivan chargers. If you make sure the batteries are at room temp before charging than the 12v chargers are superior to a HV charger with temp compensation in combo with powercheqs.
PowerCheq units don't prevent overcharging! They may help by shuffling power from a battery that is higher in voltage to the rest of the string, but they are not super efficient at doing this. They tend to pulse on and off, allowing the battery to exceed the gassing voltage for periods of time. Particularly troublesome are the two batteries on the end of the string. This is because they only have one powerCheq to transfer power from if their voltage rises too high. The entire string can develop an imbalance, starting with one battery high on one end, and the batteries progressively decreasing in voltage as you go to the other end.
Because of this, there can be a relatively large voltage difference from one "side" of the string to the other. I have found one side of a 6 battery string three batteries all going above 14.5v, while the other three remaining below 14.5v. And the end battery on the high side going above 15v, while that on the low side remaining below 14v. This was on the CV stage of charging, where the charger was at about 1 amp output.
Additionally, powercheq units do not prevent thermal runaway! You need a charger that can compensate for temperature, especially if its a fast charger.
"Shunt" type regulators are a very good option to prevent overcharging. Rudman Regulators here. They will not allow the voltage to ever go above a predefined level, but if they don't have temperature compensation than the voltage might still go above gassing voltage. Additionally, they may not prevent thermal runaway, and don't lessen the need for temperature compensation of the charger.
2. Charging each battery to 100% SOC
Again, individual 12v chargers are better than anything else for doing this. They will make sure the voltage is neither below or above a predefined level that should be compensated for by temperature, but probably won't be. As long as the batteries are at room temp, then it shouldn't be a problem.
Shunt type regulators are arguably more helpful than powercheqs at doing this! This is because they will keep the individual battery voltage below gasification, and allow the other batteries to come up in voltage before the charger reads that charging is complete via a high pack voltage.
PowerCheq units can allow certain batteries in a string to go above gassing voltage, meaning others in the string will be lower than full charge voltage, hence not fully charged while the pack voltage might be at 14.5v/battery average (at room temperature) and the charger might shut off thinking the pack is full. However, after the charger shuts off, the powercheq units will continue to try and balance the batteries out after charge.
3. Keeping Battery voltages constant.
Powercheq units are extremely useful at balancing the individual battery voltage after charge, and during discharge This is what you really shine. Neither 12v individual chargers, or shunt regulators can do anything here.
[Note: This is incomplete, will complete it soon!]
Andrew, Lets use specific model # of chargers with type size and brand of battery.
Also, temperature compensation by definition is ambient air temperature and concerns the enviroment the charger is operating in. "From what I read" Ya know, if you read it, it has to be true. "From what I read" Soneils work well in Canada where it is cooler, not so well down here in Dallas because they are not temperature compensated.
Odyssey PC 625. agm svrla (sealed valve regulated lead acid)
20 hour rate: 18ah, * 2hr: 15ah, * 1hr: 14ah, * 1/2hr: 12ah
Standby charging voltage: 13.5-13.8 volts, no current limit required
Charging voltage: 14.4-14.8 volts, no current limit required
These batteries do not heat up with discharge or during the charge using a 750 watt 80% efficient scott brushed motor and a 300 amp alltrax controller/6 guage wiring/24volt system
Soneil model 2409sr
Experience with the soneil, The charger got very hot, it took 3hrs to charge the pack when the individual battery voltage was at 12.5 volts. The chart with my odyssey manual says that 12.5 volts is a 75% state of charge (soc). soc was measured immediately after a ride, Odyssey manual says to measure soc after the battery has sat for 6 hours after discharge or charge. Voltage at the start of charge was about 28 volts and rapidly went to 29 volts, towards the end of the charge I had to monitor carefully because the voltage would start spiking to 30+ volts during the last hour of charging. After 9 months the charger quit working,, I am sure this was a heat induced failure. Nonlinear charge voltage profile, basically just jumped right up to the max voltage during the charge
Schauer model jac1212
watts: 144, each
Reconfigured my pack to 2 parallel/2 series. Use 2 schauer chargers. chargers stay very cool, charging voltage starts at 13 volts and slowly rises until it hits 14.5 volts, then within minutes goes to standby of 13.6 volts. Charge time for batteries at 12.5 volts, 45 minutes to 1 hour. Very linear voltage profile during the charge cycle
This seems like a good time to put this thread back on the table for discussion.
Some additional items
A battery charger with temperature compensation
A voltage regulator across each battery or use individual 12v chargers
PowerCheq units or other active battery balance system that works during discharge
Clean connections between the batteries, contoller and motor
Make sure all terminals and connectors are coated with dialectric grease, thoughout the entire electrical system
I can't find anything that says different. For electric vehicles, looks like you have hit the nail on the head.
I am in complete agreement with your analysis.
temperature compensated charging chart
Andrew, this website says that we are free to hyperlink the information contained within, the chart is at 9.3.3 feel free to delete what I post on this thread and incorporate it into to your original post here. ie, once I see that you have cleaned this up I or a moderator will delete my posts since the object here is'nt really a thread.
@ 2.8mv/degree F/cell
degrees F Degrees C Add to Charger's Output Voltage
160° 71.1° -1.344
150° 65.6° -1.176
140° 60.0° -1.008
130° 54.4° -.840
120° 48.9° -.672
110° 43.3° -.504
100° 37.8° -.336
90° 32.2° -.168
80° 26.7° 0
70° 21.1° +.168
60° 15.6° +.336
50° 10° +.504
40° 4.4° +.672
30° -1.1° +.840
20° -6.7° +1.008
10° -12.2° +1.176
0° -17.8° +1.344