I hope others weigh in here with any corrections, comments, suggestions and share their experience on this topic.
But IMO I would assume whatever 12V Aux battery you choose, it would be constantly kept at its float voltage by a DC2DC that has enough load amperage capacity for when all your 12V devices are all on (lights, heater fan, wipers, +more). You need to measure that, but let's pick a number out of the air, and say its a 50A draw with all 12V devices on. Then what 12V Aux battery you choose becomes a question of how much capacity will you need if the DC2DC were to fail. i.e.: it has to have enough healthy-discharge Ah capacity to make it home with a full main pack recharge. Lets pick another number out of the air, that your main pack gives a 50 mile range @65mph. 50A * (50 miles / 65 mph) = ~40Ah , factor in %60 capacity at the end of its life (40 / .6 = ~65Ah ) In this example whatever 12V Aux battery you chose would need at least a 65Ah capacity. Going with a AGM or Gel PbSO4 battery would be less maintenance, but be sure to set the DC2DC to a float voltage for that type of battery (the float voltage is slightly different for each type) http://en.wikipedia.org/wiki/Float_voltage Lead-acid battery type (12V) 13.05VDC, Gel cell battery 13.6VDC, AGM, Absorbent glass mat 13.4VDC, wet or Flooded lead–acid cell In my 15 year EV conversion driving experience http://brucedp.150m.com/blazer/ I only had a couple of times when the 12 DC2DC was not working. One of those was when I was pushing my range bubble, and was on a trip to the Monterey, CA area, which included some exploring south to Carmel's Pebble Beach (where some movie stars and other rich folk live) http://goo.gl/maps/60ygk It was a 168 mile round trip way, way back when there was no production EVs, nor public EVSE = a significant challenge for a conversion EV of its day, that most EV drivers in that area would never even think of trying. Some did not believe I had, but I had the pictures to prove it. With 6 on-board chargers, and thinking I had it all figured out I boldly went where other EV'r of that day dare not tread (kind of blows the doors off all the Tesla-trip hoopla when I had much more of a range and charging challenge). At the time I would have liked to go even farther south to enjoy Big Sur, CA, but there just wasn't any access to charging / RV parks within range. I was out to have some fun anyway, so I went as far as I safely could. My DC2DC failed on me on my return trip back home from enjoying Monterey during a Pacific coast rain storm that that came up. I was able to make it to one of my charging points on the trip, an RV park at Marina Dunes (full RV price for 1 day access). It was raining when I pulled in with the lights, wipers, and heater keeping the windshield clear from misting up, etc. I settled in for a full charge while I figured out what I was going to do. This trip was planned on one of my rare days off when hp was not working me to death (24 by 7 CE support coverage). So, I was going to try to have a happy ending to this adventure, even though the weather and my DC2DC was not helping. I will tell you what I should have done rather than what I did do, so I do not give anyone bad ideas. In my rear seat I had a gym bad full of long 12-3 extension cords, 1" pigtails, and several plugs and receptacles so that I could connect to just about any type of outlet (there was no Avcon nor J1772 at that time). Carrying around that added weight was not that big a deal when you consider my EV weighed in at 2 tons. I did not bring with me a 12V charger (back then only clunky / heavy automotive transformer type 12V chargers were available). I figured I would not need one. I was wrong. I did carry with me a pair of jumper cables, so 'I disconnected the pack (broke the 132V circuit), lifted the connections on the 12V Aux battery, and connected 3 of my 6V traction batteries (18V) to my 12V to recharge it. I would connect it for a few minutes. Disconnect it to measure the 12V Aux batteries surface voltage, and reconnect it for shorter periods to push Amps into that 12V Aux battery (trying to recharge it the hard way).' This is not an exact science, nor recommended, but it got me the juice I needed into the 12V Aux battery, so I could continue on my way back home. Today, it would be an easy thing to buy a cheap 14V 5A power supply to keep in that gym bag for such situations http://www.ebay.com/itm/14V-5A-AC-DC-power-adapter-Supply-for-LCD-Monitors-TV-/280427970150?pt=US_Monitor_Power_Supplies&hash=item414acf3e66 14V 5A AC DC power adapter Supply for LCD Monitors / TV Price: US $10.99 Buy It Now FREE Economy Shipping The above is not a charger but a power supply that would have been a cheap item to carry with me encase my DC2DC failed. Though the DC2DC rarely failed, on those long trips out into places no EV was meant to go, you need to have a plan to recover from situations like the above. The idea of using the above (in hindsight) is academic (I no longer have my beloved S-10 Blazer EV. I lost it in a 'wrong place at the wrong time' crash). So, with proper planning, you can get the right amperage DC2DC and 12V Aux battery capacity that will allow you to get back home even in worst-case situations. {brucedp.150m.com} [ref http://electric-vehicle-discussion-list.413529.n4.nabble.com/Replacing-Auxiliary-Battery-tp4670985p4671131.html Replacing Auxiliary Battery 21 Aug 2014 ] On Thu, Aug 21, 2014, at 10:17 PM, Buddy Mills via EV wrote: > I am replacing my auxiliary battery(ies)for my 12 volt system. Presently > I > have two group 31 Deka 8G31 (70lbs each) over the front bumper. They > were > left over from my old lead days before I converted to Lithium and fit the > spot at the time. My question is "What plays better with the DC to DC > converter. Gel? AGM? or just an old fashion lead auto battery? I need > something with some ump so that I can run my 12vdc to 120vac converter > (it > runs my heaters). So just a small 12v will not cut it. I also like to > trickle charge it with a small charger while plugged in and charging the > Traction Pack. Any suggestions. > http://www.evalbum.com/2887 - -- View this message in context: http://electric-vehicle-discussion-list.413529.n4.nabble.com/Replacing-Auxiliary-Battery-tp4670985p4671139.html Sent from the Electric Vehicle Discussion List mailing list archive at Nabble.com. _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
