Re: [RE-wrenches] AGM battery charging, more details

[Allan Sindelar]

Hugh, Let me give it a shot. My apologies about noting only solar charge controllers - I forgot that the sun is a rarity where you live. The return amps affects only the transition from absorption to float. If, while the voltage remains up at the bulk/absorption setpoint, the charge controller's current necessary to maintain this voltage drops below a programmed threshold, the charge controller's logic determines that the battery is full and transitions to float. That's all. Rule of thumb is that the current threshold is about 2% of battery capacity. It's a nice feature, but can bite the system owner as the batteries age and require a higher current to overcome increased internal resistance. If the absorption time has been set long (as is typical with "opportunity charging" recommended with PV systems - use all you can when you can get it) the float transition current setpoint is no longer reached and water consumption goes way up, leading to unexpectedly low water levels in the cells. Re the other question - how a charge controller senses load - it simply adjusts current during absorption and float to respond to voltage dips that are proportional to load. In other words, it tries to maintain programmed setpoint voltage by allowing more current through. The Solar Boost products actually used an external shunt option, tying to the Kelvin terminals on the shunt to measure current and allow compensatory current; a dip switch controlled internal/external sensing. Modern digital controllers apparently rely solely on voltage variations. Allan Allan Sindelar al...@positiveenergysolar.com NABCEP Certified Photovoltaic Installer NABCEP Certified Technical Sales Professional New Mexico EE98J Journeyman Electrician Positive Energy, Inc. 3201 Calle Marie Santa Fe, New Mexico 87507 505 424-1112 www.positiveenergysolar.com On 4/24/2011 11:43 AM, Hugh wrote: At 08:22 -0600 24/4/11, Allan Sindelar wrote:  if I understand your reference to transition current as a transition to float, several charge controllers have that feature. I imagine they are solar charge controllers. Diversion charge controllers as used on wind and hydro systems do not have access to the battery current to measure it. I am not sure how the solar ones can 'know' the battery charge current without knowing the load current (to inverter) but maybe there is a controller with an external shunt or something?  I'd like to hear about that. thanks for any further pointers...

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