I am glad you brought up the topic of battery tending and charging as that is something I have looked into as well. When I was talking to the folks at Concorde and I asked about the best way to keep the battery "topped up". My concern is that lead acid batteries (whether flooded, gel, or AGM) have their best life if kept fully charged or moreover not left undercharged for long periods. The reason for this is a battery is really just a chemical reaction. As charge is taken out of the battery there is a flow of electrons and a conversion between the cathode and anode plates and the result is lead oxide being converted to lead sulphate. Please don't quote me on that last part - I read a very good write up on the web a few weeks ago regarding the science behind lead acid batteries - if you Google I am sure you will find it but the bottom line is that one of the by-products of using charge from the battery is a build up of lead sulphate (and that bit you can quote me on
).The lead sulphate that forms is converted back to lead oxide (I think) when the battery is recharged so no big deal... except if the lead sulphate sits for a while (ie undercharged battery sits for a few days in the hangar) then the sulphate begins to crystallize and once that happens you have permanently lost that much of the battery's capacity. This is an over simplification but the bottom line is that lead acid batteries will keep their capacity longer if kept fully charged.
So my concern was landing with a nice full charge and then having a long taxi back to the hangar (esp at night) mostly draining the battery due to the low output of the generators at low RPM and then putting the plane away with an undercharged battery.
So in talking to the folks at Concorde I learned all the things listed in my previous posts but I also asked about tending the battery for the concerns as stated. I specifically commented that I know they recommend Battery Minder as a brand and that they worked with Battery Minder specifically to make an optimal charging solution. Here is what I learned there....
Skip told me that the Battery Minder folks had been great to work with and yes - they did optimize for the Concorde AGM and they said that the Battery Minder is a very good charger. They did extensive testing with it and very time it performed extremely well. But he also said there is "nothing special" about that specific charger. He said that any charger is fine so long as the charge rates are correct. He commented that the Battery Tender was the first group they tried to work with because they found a lot of batteries having problems being over cooked on Battery Tender chargers (too high a charge rate). So they tried to work with Battery Tender but he said for reasons he did not know that process did not work out.
So here is the key... the correct charge rates are as follows according to Concorde regarding their AGM batteries:
Bulk Charge: 14.2V +/- 0.1V
Float Charge: 13.3V +/- 0.1V
The Bulk Charge is what is applied when the charger senses the battery is depleted and needs to be recharged. The Float Charge is what is applied when the charger senses the battery is already pretty much charged and just needs to be "maintained" in a topped up state.
Any higher than these rates (+/- 0.1V) and the battery will overcook. It won't blow up or anything like that... in fact Skip said they had done extensive testing trying to get them to burn - he even held a blowtorch to one but he said the are very safe and won't burn. That said these "sealed" batteries are in fact Valve Regulated Lead Acid (VRLA) batteries. They have valves to release pressure if over charged and when that happens some of the moisture in the AGM battery is vented to the atmosphere (not a leak like a flooded battery - you would never visibly see it) and the net result is the battery looses capacity. That was what was happening when they approached Battery Tender about the problem.
Overall Skip said as long as your charger puts out the rates above then it's fine to use. I myself bought a Schauer charger with a Piper Cannon plug before I learned all this. My Schauer charger has a Float Charge right at 13.3V which is perfect but the Blulk Charge is too high. The great folks at Schauer said they would be happy to recalibrate it for me for free if I sent it back and they appreciated the info on Concorde AGM specs (and I also relayed this back to Skip. My charger rate can be adjusted internally by a pot on the circut board but they also said you cant' just hook up a multi meter and adjust it. He said they have $10K of equipment to benchmark the charge rate so send it back and they will calibrate to whatever spec I like.
However.... my charger is still of limited use even calibrated because you can't maintain a battery and shouldn't really even charge it through the external plug. To charge properly there can be no electrical consumers in the system... ie having the master switch on running gyros etc... and charging at the same time won't work well. It's fine to hook up as a GPU for doing avionics database updates etc... but this is not the way to charge (bulk or float). So the only option is a direct connect to the battery and I know a lot of people do it but it is really not legal and Concorde confirmed that. Skip commented that even the connectors that come with the venerable Battery Minder charger are just "standard lamp-cord" (his words) and not aircraft grade cables. To install a remote charge point, proper wiring would need to be added and some kind of field approval. That or open the nose and battery compartment every time you want to charge. So for now I am not going to worry about it - just fly a lot and maybe bring the RPM up at the hangar for a few min before shut down to top the battery.
Anyway that's what I have to share on that topic. Sorry if this is a bit of a hijack of the original purpose of this thread but it's related.
- Charles
