Since I've gotten no reply from GreenSaver I pose these questions to the tinkerers on this forum about the (SP27-12) or (SP36-12):
When charging in constant amp mode what is the idea amps GreenSavers like to be charged at?
When charging in constant volt mode what is the ideal voltage formula (temp compensated)?
What is the ideal float voltage?
Can they be floated indefinitely with no detriment if the current is low enough?
Goals (in no particular order): Keep batteries in a string balanced, fully charged, and never over charged for an indefinite time. Keep costs to a minimum. Make it small enough it can be stored inboard. Provide constant desulfation. Provide the longest life for these batteries as possible. Keep project open source, with socketed microcontroller for firmware upgrades. Temperature compensated charging. Negligible battery drain for long term unplugged storage. >5 year service life with daily use consisting of 8 hours of operation. Avoid patent infringement based lawsuits. Upgradable for management or more or less batteries / other chemistry.
Plan A: Build a pulsed constant current string charger with individual shunt voltage regulators on each battery.
How it works: A transformer (rewound microwave oven transformer?) that saturates (at a to be determined amperage) provides current limiting. The transformer's output gets full wave rectification then smoothed. Mosfets pulse the charging current at an optimum battery absorption rate (variable rate?). The batteries soak up the amps. The batteries one by one reaches their charging voltage ceiling and the shunts engage limiting the duration of the pulse the charger receives, then disengage as the battery voltage drops below the voltage ceiling (voltage ceiling automatically adjusts with a thermistor circuit, thermistor also stops process if an overheating condition is detected). Once the transformer reaches an upper voltage ceiling for a preset length of time it shuts off and waits for the self discharge of the batteries to fall below their fully charged resting voltage (temp compensated) and begins again with step one. This will keep the batteries topped off without ever over charging them and keeping costs to a minimum. A green light will illuminate saying it's fully charged. I believe this strategy will work with any sort of battery technology too, but haven't done extensive research into it.
Notes: I went with pulsed current, because my online research has shown it speeds charging of lead acid batteries significantly. The space between pulses allow the chemical reactions inside the cells to complete. Another benefit to is to it are thermal. Higher current can be put into the battery without causing the internal temperature and pressure to build (GreenSavers have been charged at 10A without much heating, but it's as of now uncertain if the silicon was insulating the heat to a large degree if only GreenSaver would answer e-mails). The shunts will be cooler too with time to dissipate the heat. If there is ever an small overcharging condition and an air bubble forms, (in AGM batteries at least) they are designed to recombine the gas during the drop from charging voltage to resting. The shunt's switches are controlled by a 14bit ADC monitoring each battery's terminal voltage. This will additionally allow for active monitoring of each battery so to shut down if there are any serious problems during charge, and all kinds of fun things like a cheap LCD to tell you the things PakTrakr can + whatever code people add to it.
Question: If shuts are used it will surely provide overcharge protection, but will it mean that other batteries get undercharged? According to USATracy the answer is yes, but I'm not convinced.
1) http://www.nrel.gov/vehiclesandfuels/energystorage/pdfs/evs_17paper.pdf - Charging Algorithms for Increasing Lead Acid Battery Cycle Life for Electric Vehicles
2) http://www.ipenz.org.nz/ipenz/publications/transactions/Transactions98/emch/2wilkinson.PDF - A new pulse charging methodology for lead acid batteries
Use stock string charger and use 1 giant ultra capacitor with a switching algorithm to take power from the strongest battery and put it into the weakest. Monitor each battery voltage with 14 bit ADC to find the strongest and weakest battery. Shut off the charger if an overcharging occurs, turn it back on once batteries are better equalized but not yet charged (voltage compensated). Place capacitor in parallel with controller power input for boost in acceleration. Display battery information output on LCD.
Benefits: More simple design. May extend the range since it's limited by your weakest battery.
Drawbacks: Possible lawsuits due to patent infringement. Loss of 10% of the energy you move from one battery to the next. No desulfation.
Build the charger from plan A, and the single capacitor equalizer from B.
Note: Shunts are cheaper than capacitor equalizers ~$8 vs ~$20 per battery respectively
This post took way too long to type. 5hrs. It's the result of my first week of obsessively researching this stuff.