Getting 36v from a 12v battery

Thanks Dan, I looked at them a few days back and a bare converter from Mouser was around $72. Makes the $125 surplus unit look pretty affordable. I am currently going with the 3 x $15 panels from HF and see how that works.

Plus, the little solar panels are intrinsically current limited to a safe trickle-charge level. Further, while batteries can be used in series to drive a load it is best to charge them individually. Batteries are commonly used in series for 24-volt and 36-volt electric trolling motors. Good chargers for these have separate isolated outputs to charge the batteries individually without disconnecting them or overcharging any of them. The difference between a "good" charger and a "not so good" charger is at least a couple of years of battery life. My last set lasted more than 5 years.

The charge current from 5-watt solar panels presents no significant risk of overcharging at about 350 mA or so.

A little inverter with separate outputs and charge controls running from the larger solar power source would be better if higher charge current (faster charge) was necessary, but that does not appear to be the case here.

Said little inverter would probably cost less than $45 for parts, but it would only make sense here if one wanted to build a little inverter for the helluvit. It'd cost me zip because my goodiebox would supply the parts, but I'd spend a day doing it -- and enjoy the activity and maybe learn a thing or two.

Go with the little panels, Jenny. There's a lot to be said for simple. If that doesn't work then we'll devise something that does, but I'd bet it'll work just fine with 340 days of sunshine per year.

The electronics don't have to pull the load, the batteries will do that. Your alternator wouldn't start your car even if it were spinning, right? The battery does the short bursts of heavy lifting.

Elex (or small solar panels) supply small steady current to replenish batteries that supply heavy current very intermittently.

Low power boost circuits typically use a flyback topology, but I'd go with a foward converter here so peak supply current would be about the same as steady-state current. LM3524,5 or 6 running at 100 KHz, coupla $2 MOSFET's, a ferrite xfmr with 3 secondaries that'd easily fit in a shirt pocket, etc.

You didn't rerad the rest of the post - the charge controller only needs to cost about $10 if you buy all the parts new. A good scavenger could likely do it for $2.(or free)

Say again? A battery is a string of series connected cells. A 12 volt battery is sic 2 volt cells in series. You don't charge THEM separately. As long as the discharge in series, you charge them in series. If anything causes one to be discharged at a different rate than the rest, or they are of different capacities, individual charging is adviseable. However, when discharging in series, so all cells see the same amount of current, charging with separate chargers, particularly un regulated chargers that may not be evenly matched in output (read "solar cells") you could easily end up with one battery less charged than the rest, and then discharging them could damage the lower charged cells by reversing them. The rule of thumb is charge batteries in series, and dischrge them in series. If you need greater current capacity, buy bigger cells instead of parallel connecting smaller cells. That's why it is better to use two GC2H golf cart batteries in series than two 12 volt batteries of the same physical size (rougly group 22) in parallel.

And not terribly efficient. With solar power, you want efficiency.

Only because it would be impractical to do so. The cells in a particular battery are about as identical as they can be because they're manufactured at the same time in the same way from the same lot of materials -- and even then they are sufficiently different that usually one cell fails before the others do. Different batteries with different ages, capacities, brand or even just with different history will accept charge differently. Therefore, to charge the lowest one fully in series you must overcharge the others. Overcharging is the worst thing you can do to a battery.

Other way round. For identical discharge amp-hours (by definition in series), the batteries may need slightly different charge amp-hours to recharge fully but not overly. The little solar panels won't overcharge because their current drops as battery voltage rises with charge, and even their max output current is a safe trickle or maintenance charge level.

Don't know what assumptions that assertion is based on, but it is not how it's done in at least one practical case: trolling motor batteries.

Check the setup in any pro's bassboat (Ranger, Triton, etc) with a

24-volt or 36-volt trolling motor. They use the best gear (including chargers) available (albeit sometimes limited to what's available from their sponsors) because they make their living in their boats and they need their stuff to work reliably all day every day upwards of 200 days a year on the water including competition days. Their chargers charge each battery individually. Accessories (sonars, GPS, power trim, livewell pumps, running lights, etc) are run off the engine's battery; the TM batteries only power the TM.

Yes.

90% efficiency is not unusual even in a small switchmode converter. If diode rectification (rather than synch rect) is used then efficiency will be a bit lower, but Schottky diodes are inexpensive, readily available and commonly used.

Why do you want efficiency in a solar battery charger if it can get the job done? Sunshine is free.

Next to "unequally yoking" them. Batteries, whether series or parallel connected, SHOULD be as identical as possible. Mix and match doesn't cut it.

And series charging satisfies that requirement as the resistance of the cells changes with state of charge. AN "equalizing charge" brings them all back to full .

The exception, rather than the rule. In Electric Vehicle applications, they are series charged with a charger of the proper voltage. The Bass Boat guys use the 12 volt chargers for the very reason you stated - because they are available from their sponsors. The low performance consumer trolling motors are 12 volt - so that's what the sponsors supply, stock, advertise and sell. There is no good engineering reason to use separate chargers. (at least from all the reading and research I've done in regards to electric vehicle applications and large Uninteruptible Power Supply applications. Even Telco battery sets are series charged at the bank voltage, not individually. ANd those things go for decades.

Primarily, up here, because the sun doesn't shine all the time, and the power output of the cells is not terribly high. If it's cloudy and rainy for a few days, you still need to get in and out through the gate. Also, big panels are not free!! Little panels means little power as stated above.

Worked on a solar electric home and office setup in West Africa. You didn't waste any more power than absolutely necessary during the rainy season!!

Both MinnKota and Motorguide offer 24-volt and 36-volt trolling motors, and that's what is found on high-performance tournament fishing boats. These motors are about as high-performance and efficient as DCPM motors get. They're electronically-commutated switchmode-controlled multipole motors with substantial rare-earth magnets. They'll pull a boat thru salad that is almost too thick to pole thru. They compete on performance first, then price.

MinnKota also offers excellent dual- and triple-bank multi-stage microcomputer-controlled chargers. Unlike some of the "consumer grade" chargers ala Wal-Mart, they work very well. They are often integrated into the boat. It would be cheaper and simpler to make a single 24-volt or 36-volt charger with a single charge control -- but that isn't what they do and this isn't because they don't know what they're doing.

Individual controlled charging is the more general case. If the batteries are truely identical such that series charging would work well, then the charging processes (series vs individual) would be indistinguishable. The current vs time in each battery and the total voltage of the stack would be the same either way. So series charging can't be "better", it's just cheaper and adequate under the best of conditions.

Jenny has 340 days of sunshine per year. The "right solution" may well be different in the Yukon or a rainforest. Her batteries will probably open and close her gate quite a few times between sunny days if necessary at about 0.133 ampere-hours per operation. "Little power" is all she needs on average.

Hi Clare,

That is beyond my electronic capabilities to design much. I am comfortable at building kits, fault finding etc and I have designed some simple DC motor controls at times using Gottlieb's books. I did a search for charge controllers and came up pretty much empty in the 12v to 36v range..

Would you care to provide a schematic that will work? Maybe a website or two where I can get started?

I don't want to get between you and Don on the argument of series v parallel, charge/discharge, way out of my depth. But I do like the idea of a single 36v controller instead of the 3 HF units. I was planning on the HF units as the most simple solution I could handle. But a 10-buck 12-36 charger sounds good and I do already have the 5W panel.

Thanks

Hi Don and Clare,

I think that's the crux of it. I do have an abundance of sunny days and even those days that are not full sun, we still get a few hours. The opener will probably be only used twice a day. Once out and once in. Probably for less than 75% days of the year as I don't go out every day. Trickle with some waste would be fine.

OK, guys, anyone want to toss a schematic on their website? I wont be doing the battery side until later this week. I will test it with a spare car battery at 12v, albeit slow, then the 3 batteries and charging side will come later.

Hate to say it, but the chances of the batteries being closely matched to each other is slim. LOL Using what I got.

Thanks guys.

I don't think Clare's 10-buck controller included conversion from 12V to 36V.

I think just a little inverter or charge pump would suffice, because there is about zero risk of a 5-watt solar panel overcharging your batteries. It's intrinsically current-limited to begin with.

5 watts might be marginal if you use the gate more than a couple of times a day.

Perhaps Clare will post something. I'll try to also but I gotta get my taxes done first. Check the thread from time to time. I'll post here rather than Email in respect of your preference for peer review. Perhaps others will as well.

jk

It could be useful to know a bit about how your 5W panel behaves. If you have a VOM (multimeter), get readings of open-circuit (no load) voltage and short circuit current in typical sunshine. For short circuit current, just set your VOM on DC amps, connect it to the solar panel and read the meter. You won't blow anything with a 5 watt panel.

It might be considerate to try that with your own batteries before advising someone else to do it with theirs.

On Sun, 01 Apr 2007 15:17:52 GMT, snipped-for-privacy@msn.net wrote:

Dead simple. Decide what your charging voltage should be. I'd say 42 volts. Get a heavy duty 42 volt Zener diode (capable of handling the maximum output of the solar cells). Get a silicon diode , 50 volt PIV and heavier than the maximum output of the solar cells. Get a power resistor that will handle the charging current. One and a half amp panels? That means you need to pass 1.5 amps with negligible voltage drop. E=IxR, so 1.5*.7 ohms = 1.07 volt drop. when charging. Should be doable. Now, what happens when the Zener goes into regulation mode, and clamps the output to ground. assume, for worst case scenario, that the solar cells are a "hard" supply (they are not) at 42 volts. .7 ohms and 42 volts - i-e/r, or 42/.7= 60 watts. A bit heavy - 60 watt Zeners and power resistors are pricy, so back to the drawing board and try a 1.5 ohm resistor.1.5x1.5= 2.25 volts. Should be able to handle

2.25 volt drop at full load, the voltage drop will diminish as the charging current drops. 42/1.5= 28. A 30 watt 1.5 ohm power resistor, or something reasonably close, should be fairly easy to find in a wire-wound ceramic surplus, and a 25 watt (2 amp +/=)zener should also be do-able. Now, connect the - of the solar panel string to the - battery terminal of the 36 volt string, and connect the resistor to the + of the solar panel string. Connect the Zener from the other end of the resistor to ground so that when the voltage gets over 42 volts it pulls it to ground. Connect the silicon diode from that same connection to the + of the battery string. The silicone diode prevents the battery from discharging into the solar cells or shorting to ground through the Zener. The Zener prevents the voltage from excedint 42 volts by clamping the output to ground, dissipating the excess power in the zener and the resistor. In the real world, with the solar panels not being a 'hard" supply (having significant internal resistance themselves) a 15 watt resistor and zener will likely be more than adequate. In the real world, 42 volt Zeners are not a standard - 43 volts are,The charging voltage will actually be about 42.7, more or less, and because of the forward drop of the isolation diode,using 2 diodes in series would give you about 42 volts The old ECG number for the Zener would be ECG5206a for a stud-mount 10 watt, or ECG5272a for a 50 watt. A 43 volt 3 amp zener would be a 1n5260 series - 10 for $1.15 from DigiKey

The isolating diodes would need to be more than 2 amp units to be safe. Mount on a heat sink to be safe..The isolation diode can be any ecg5800 series (1N5400 series)

The resistor, in the real world, could be anything from about 0.7 - 2 ohms and 10-25 watts.

Ebay has Item number: 120103830342, 20 3 watt 2.2 ohm resistors for $5. 2 in parallel gives you 1.1 ohm at 6 watts. 3 in parallel is .7 ohms at 9 watts. - just for an example. Or Item number: 120103830367, 20 4.7 ohm 3 watts for $5. 4 in parallel is 1.17 ohms and 12 watts. 5 in parallel is 0.94 ohms and 15 watts.

Looking a bit farther, Item number: 110110012402 is 20 of 1 ohm, 25 watt. Mix and match to your hearts content- you can have half ohm, 1 ohm, 1.5 ohm, 2 ohm, 2.5 ohm, whatever you need with lots of power capacity to spare. Likely get them for $5. (and you don';t need to go to e-bay and buy large lots if you have a local surplus depot or even electronics supply house close by).

Remember, if you find surplus parts that are way over capacity, but cheap - use them!!!

I've seen 150 watt resistors, 1.5 ohm, for a buck. I've seen 300 amp

600 volt diodes for a buck. The only part you might NEED to buy new is the Zener.