Recharging 12V NIMH Batteries from a Solar Array

Options
Every summer, I make a multi-day bike trip where I carry camping gear on my bicycle. I also have sleep apnea, so my gear contains a CPAP machine. I have been running the CPAP machine off of a pair of 6V 10AH NIMH battery packs (wired in series) based on D-cells. The setup works great, I can get 2 nights of running out of the battery packs. I have been charging the battery packs from a small 120VAC smart charger, but this means camping somewhere that has an AC duplex for me to use.

I would like to plan bolder trips, so I was thinking of adding a small solar pannel to my kit. Actually my plan is to drape a solar pannel over my rear panniers and try to charge the NIMH batteries while I ride. Wired as a 12V pack, the batteries should start to charge at 14V. Impedience wise, this seems like a near perfect load for a solar array. If I can get a sustained current flow of 1A out of the array, then I should be able to recharge the battery pack in 8 hours. I have been looking at arrays ranging from 13.5 to 26 watts.

So what do I need to charge my 12V NIMH battery pack from a solar array?

Do I need to worry about impedience matching?

Do I need a charge controller?

Can I trust published specs from the PVA manufacturer?

Are their questions that I should be asking?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    If your charge rate is C/10 or less, generally, you can simply connect the solar panels directly to the battery pack without problem... If did this 24 hours per day x 7 days a week, the battery pack may only last something like a year +/- vs several years with a charge controller.

    But, you are only charging it with a solar panel (4-6 hours of full sun equivalent per day)... So I would not worry. Just use the correct AC battery charger when at home (and even then, for better life, only connect the charger once a month for a day to keep the batteries charged if you do not have a "float" stage on the charger--many AC chargers tend to overcharge the batteries too if left connected 24x7).

    Power wise, it is not unreasonable to derate the panel + charge controller by 0.77 (panels have "marketing" ratings--which tend to be over optimistic in hot weather and also to account for other losses).

    Recharging NiMH battery packs tends not to be very efficient--Perhaps 60% efficient--but it is pretty variable. A lot depends on the exact current you supply to charge the batteries vs their AH capacity.

    You could use a $60 DC Amp*Hour meter like one of these to measure how much it takes to charge the batteries after each use.

    Also--Solar panels need to be free of shade and pointing at the sun--so during your ride (assuming this is when you charge), you will need to mount it so that panel is pointing relatively at the sun.

    If you charge while camping for a few days--the panel should be well anchored to prevent it blowing over in the wind--The panels are tempered glass. Pretty tough, but once they get hit wrong the panel will shatter like safety glass.

    So, to estimate panel size lacking any more data:
    • 5 amp*hour per night * 17.5 volt panel rating * 1/0.77 derating * 1/0.60 batt charge eff * 4 hours min sun per day = 47 Watts minimum
    The above is a guess at what wattage rating you would need for a successful system.

    If you are cycling only in summer in the US southwest (or other "sunny" region, you might want to assume 6 hours of sun per day). The battery pack you have may be more efficient than I have guessed.

    And do you need a backup (say you have a few days of clouds--what happens if there is no AC backup power). Should you bring a DC clip on charger and ask if you can use somebody's car to recharge?

    Regarding charging without a battery charge controller--You could use a solar charge controller to limit voltage and/or a temperature sensor in the pack to cut the power/alert you if the pack is getting too hot to help reduce the chance of overcharging. Everything adds weight and complexity.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    Horrible Freight has a nifty 2 stroke 500W inverter generator ! Lightweight for charging.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • Ralph Day
    Ralph Day Solar Expert Posts: 1,019 ✭✭✭✭
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    And no hernia when the next site camper hurls it into the bush!

    2 stroke, what a dirty way to go:grr

    Ralph
  • vcallaway
    vcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    I just had flashbacks to the old bicycle lights that used a little generator on the tire.

    Wonder how something like that would work?
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    I think those side of tire alternators were ~6 watts at 6 volts or so or about 1/2 amp at 12 volts and 10 hours of peddling (or 60% more) to recharge after a night of use. And it would be like peddling up hill the whole time. And the sidewall generator (probably alternator) would only last 100's or a 1,000 miles or so...

    There are generator hubs--If they were 2x as efficient--that would still be a lot of peddling/load for a night's use. I guess the new ones are rare and expensive... You may find some older ones (late 60's or later?) that work.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2
    System2 Posts: 6,290 admin
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    Lots of good responses here.

    1) I find that for every 4AH of power that I take out of the NIMH batteries, I have to put 8AH of power back in to recharge the battery.

    2) Yes, a power meter could make manually managing recharging possible. I would have a clue about how much I have power I have put back into the battery pack.

    3) I am old enough that I remember those dreaded side mounted generators. The wear and tear on the tires is awful, and it took so much work to run them.

    4) Hub generators are still made. Shimano in Japan makes one, and Schmidt in Germany makes several. These are intended to drive 6V LED lights, so the amperage is not that high. I think these are capable of producing 3 watts of power, not enough to charge my packs.

    5) Wow, 47 watts of power needed. I don't think I can drape a panel that big over my panniers.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array
    5) Wow, 47 watts of power needed. I don't think I can drape a panel that big over my panniers.

    That's where the expense of the high efficiency panels may pay off. Offhand, I don't remember the mfg's name.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • vcallaway
    vcallaway Solar Expert Posts: 157 ✭✭✭✭✭
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    I've seen some flexible roll-up kits for backpackers. Problem is they are anywhere from 5 to 15 watt and expensive.

    I have a couple of battery maintainer panels that sit on the dash. Those are about the size of a standard keyboard and they are only about 5 watts.

    You might also look for a 12v CPAP. I know a couple of RV'rs that use them because of the lower power needs.
  • System2
    System2 Posts: 6,290 admin
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    I am using a 12V CPAP machine. There is no internal transformer, so the unit is both light and compact.

    But it still draws 0.5A.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Options
    Re: Recharging 12V NIMH Batteries from a Solar Array

    the trouble is you would need 4ah delivered to the batteries and that assumes 8hrs of run time so that means at least 1a imp from the pv to start with for 4hrs of full sun (may be pushing it as you often will have a pv flat or not optimally aimed) it would be a standard controller or linear regulator utilized as it is a small operation. as with most using standard or linear controllers the upper voltage is wasted, but necessary to insure a good charge. if say at 17v (note here that vmps are usually a bit higher) at 1a then this is a 17w pv minimally. adding say 25% more for possible oversleeping by 2 more hours brings that to 21.25w. i'm not too sure on the charging losses for the nimh batteries and other losses, but this can possibly add a significant amount more to the pv already at 21.25w. adding in the bad angles, charging losses, clouds/rain, etc and this could be possibly doubling that to around a 40w pv. is it worth it? only you can judge that. is it portable enough for you? probably not, but if you want to go where there's no power with an item like that to be powered, you do what you've got to do.