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Solar Power Tech Question


ThePog

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Yo, some you you geeks here might be able to help me out with some advice, @h4nd maybe?

There are a couple of things i want to do with my truck project;

1. There is a 24-12v DC converter that is powered all the time for 12v stuff that needs constant power. In my head this will mean a low level draw on the batteries all the time. To counteract this i want a solar trickle charger to keep things maintained.

2. I kindof want to fit a little Waeco thermoelectric fridge. I know these are not the most efficient way of cooling or heating, but I thought potentially this could run off whatever solar stuff there is for the charging to reduce the load on the batteries.

https://www.marine-deals.co.nz/fridges-freezers/dometic-coolpro-tc-14-portable-cooler-and-warmer-14l?gclid=Cj0KCQjwpLfzBRCRARIsAHuj6qWfh9Xj9fkXAblzvmD9QWgCPuz8AlB-Jo0KGOx0v1OD5g5kxMgnyFQaAuqIEALw_wcB

64W/3.8A

This one is a compressor fridge, much better

https://nz.adventurekings.com/15l-centre-console-fridge-adventure-kings.html?gclid=Cj0KCQjwpLfzBRCRARIsAHuj6qWtbgm8ghuMKQ9qFTA2VfeBWpJKEjlNGI68OQjRPjdr5BZtK1UQjB8aAmQ7EALw_wcB

1.9A@24v

Bear in mind the truck is all 24v.

So what i want to know is can I just set up a 36v solar panel and charge controller and use the truck lead acid batteries as the storage? ie mostly use the solar system as the charger, but use the load terminals on the controller to run the fridge...

This is basically to save the weight and cost of 2x batteries that would normally be dedicated to the solar setup.

I also have a 12v or 24v load switching relay that can automatically deal with sharing power between the house and start batteries if so required.

Do I need to isolate this system during starting?

Will the truck batteries deal with running the fridge overnight and still work the next day?

Am I an ignorant fuck who has no real appreciation for the issues surrounding doing something like this?

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Sorry, had a brain fart, just remembered this.

1. Yep, good stuff, lead-acid goes bad when left discharged.

2/ yep, later in the arvo, (more below):

Yeah, 36V panel (which is probably open circuit only, it falls when you load it up - the regulator sorts that out), and some sort of regulator to drop it to 28.8V (charging Voltage)

It all just comes down to Amp hours, really, if the fridge does what it says on the tin, your pair of 12V 100Ah (e.g, you didn't say) batteries will run the thermo-elec for 100Ah / 4A = 25 hours. Then of course, your battery is dead and you can't drive home.

If you want to charge that all in say 6 hours of (full) sunlight, you'll need a 16A charger so 28.8V x 16A is 460W input + losses.

Lot's of people set them up this way,  BUT,  doing that will fuck your batteries in short order:

If you fully discharge lead-acid, the lifetime (cycles) plummets. 

  • at  100% discharge ~200 cycles (deep cycle battery), 50% only discharge gives ~450 cycles, 30% gives ~1200 cycles.
  • You'll actually only be able to poke 70% recharge in with ideal conditions in 5-7h (bulk charge), then the battery charger changes over to soak charge, then the battery needs 12 - 16 hours to finish charging.
    • Con: Never happens in winter / forest
    • Pro: During that soak charge time, you now have excess capacity (if in full sun), so fridge etc loads will run direct off solar. If it's a sunny day.
      • Con: Rain, fog, cloud, you could be looking at 20% solar output opnly.
  • Every now and then (month or 2) you're going to want to do an equalisation charge (16 - 24 hours) to make sure all the cells top off.
    • This means putting more solar on the roof doesn't help (the batteries charge faster, it just takes time)
  • So you @yoeddynz living off grid needs 4 times as much solar as you think (rainy days), and 3 times as much battery, or your batteries will degrade fast
    • You can't charge them fully, so if fully off grid, they're going to degrade anyway.
    • You'll get away with it for a while in a bus, of course, if you have enough not to go flat, and can go home and soak charge.
    • if you want to ensure very reliable power availability, even more $$
  • Of course, there's whatever else onboard there for loads (stereo, laptop?) and, charge/discharge and  conversion inefficiencies (prob ~90% each), add them all to the Ah (Amp hour) tally.
  • So: what you save on the fridge, you're likely to pay way more for in extra solar and storage capacity needed.
  • Use a LVD (low voltage disconnect) to make sure gear doesn't over-discharge your battery (and fuck it). Often built into the charger, or cheapish separate.

Probs don't need to isolate all that junk when starting, all those loads are small ( fridge, 4A) compared to starting (200A for 3.6 seconds [ i.e. 1/1000 of an hour] is 200/1000 = 0.2Ah). They just add up over time (4 Amp Hours, 24h = 100Ah)

Simples, and ugly.

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Cool, a no to the fridge unless I have extra batteries..

Its interesting cos I have a solar setup in the bus, 300w panel, 130ah AGM batteries at 24v. This runs an 80 litre compressor fridge, led lights, water pump and occasionally a 300w inverter and never gets below 88% charge overnight no matter what. Probably helps to live in Nelson I imagine.

I kind of guessed that lead acids wouldn't be great, so thanks for confirming...Chur.

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How many Ah in the battery in the truck? 

And how many load amps for the compressor fridge (same 1.6A?). Bus sounds good, and only discharging to 88% will help keep your batteries healthy. In practice, you're getting away with it in the bus, Im'm just showing a worst case, full use, off grid situation. If you can manage the power drain  ...

wait  a minute -- click! Whats the duty cycle on the fridge when it's sitting there closed? 25% running time, maybe?

 

AGM are a flavour of lead-acid, it's just Wet lead acid are optimised for high cranking amps rather than deep cycle. but 100Ah is 100Ah.

You may find you'll get away with it fine with a set up like your bus, just want to watch it close for the first while.

 

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I don't have anything insightful to offer on solar, as I use 100AH lithium, redarc bcdc charger and 200W panel for futureproofing.

Lithium Iron is at a price point now, that they are better value for money long term than AGM.

 

 

Here's some more fridge options though. I see there is a Jaycar reseller in Nelson if you wanted to poke them IRL.

https://www.jaycar.co.nz/search?text=brass+monkey&CSRFToken=08de16a6-2294-4871-b8c9-5615c862547a

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Can't remember what ah the truck batteries are, they are pretty chunky and new.

The bus fridge runs at about a 15-20% duty cycle, it's actually amazing how little power it uses for the size. It has a wee freezer compartment as well.

The bus solar setup is completely independent of the starting batteries, I did have that relay to fit, but I never found the need for it.

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13 minutes ago, UTERUS said:

I don't have anything insightful to offer on solar, as I use 100AH lithium, redarc bcdc charger and 200W panel for futureproofing.

Lithium Iron is at a price point now, that they are better value for money long term than AGM.

 

 

Here's some more fridge options though. I see there is a Jaycar reseller in Nelson if you wanted to poke them IRL.

https://www.jaycar.co.nz/search?text=brass+monkey&CSRFToken=08de16a6-2294-4871-b8c9-5615c862547a

I'm going to jaycar tomorrow anyway, might have a sqizz.

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Ah, there you are, I floofed the demand calc. 20% * 3.8A * 24h, etc

@Mof Lead acid is cheaper per nameplate A/h, but @UTERUS is right for the long term:

  • LiPo can be discharged more (more usable capacity per Ah)
    • and last longer (more cycles),
    • And they charge a bit faster (hours), resulting in a better match to solar charged.
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@ThePog given how well the bus copes, and if you're running the truck motor everyday, your fridge in the truck would probably be OK on the existing batteries. Just watch it close the first couple of times, and you wanna turn it off if you're not using the vehicle for a few days.

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