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Charging 36v from 24v boost report

DThames

Solar Wizard
Joined
Nov 22, 2019
Messages
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I have the need to put some power into a 36v golf cart battery at a remote location but not tie the cart to a solar charger during the daylight hours. I already have an investment in some 24v portable battery packs and emergency power boxes. I purchased a boost converter with CC/CV output. I did the first test last night and results were good.

I had a shut meter in line on the 24v side and recorded 968 watt-hours out of my 24v LiFePo4 pack (64, 32700 cells). 24 amp hours went into the 10s Li-ion golf cart with the battery voltage about 37v, showing a conversion of about 92%, just looking at (power in 36v/power out 24v). The 24v pack I was testing with has a 30 amp BMS, so I limited the load, pulling about 20 amps from the 24v to deliver about 12 amps to the 36v battery. The boost converter has an adjustable low voltage cut off, to save the source battery from going into BMS cut off. I set it to drain down to about 3.05v per cell for my LiFePo4 pack. The low voltage cut out causes the current to approach zero as the battery voltage approaches the set point, so is not like a digital "off" set point. It slowly turns off and is a bit confusing to adjust. The adjustment point, indicated by a red LED coming on, under no load is not the same as under load, or so it seems. Adjusting by test cycling right above the desired cut out works fine.

The need to do this type of charging is for a multi-day deer camp trip where we have no AC and don't want to be running a generator. The camp location is on our land and we have a permanent camp there, but this camp is hours from home. So no running back to the house at night for AC power. I have a couple of used 60 cell solar panels that I can get about 400 watts from (total watts in full sun) that will be mounted and left at the camp. I can show up with my battery box (built in solar charger) and charge my 24v battery during the day. At lunch time or at the end of the day, we can dump the 24v battery to the golf cart to keep it with a good charge. The golf cart is new to this use for us. The area is pretty hilly and the cart battery is not large. It has a solid, usable, 80 amp hours but on these hills with a load, the amps can go up in a hurry. When the battery is low, a high amp challenge can make it take a nose dive to a cell low voltage cut off, so I want to keep it in the solid voltage range while we have it at the camp. Adding 24 amp hours to a 80 amp hour battery is a reasonable boost. I think the cart battery would last us 3 or 4 days without the boost but with the boost, we should be in great shape. So, good test. I need to get the boost converter mounted in a box next.



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Good to know that it works.!

Just wondering if there is any long term damage from using this? Is this CC/CV technically?
 
Good to know that it works.!

Just wondering if there is any long term damage from using this? Is this CC/CV technically?
Yes, CC/CV (Constant Current/Constant Voltage). I set the output current for 12 amps, which is held until the voltage of the battery climbs enough that the voltage output of the boost board and the voltage of the battery are close enough to the same that the current drops off as the battery voltage come up to the same as the output. The current then approaches zero as the battery reaches a steady state.

For the application describes in this thread, it is most common that we dump some charge to the cart battery but will fall short of fully charging it. One nice feature of this board is the low voltage cut off so it doesn't drain the source battery to BMS cutoff. I don't think these boards like to have power on the output and no power on the input, so that feature is a bit of self protection in that regard.
 
You have about a 1kw pack? I guess 400W could charge that in one day. Sorry for thinking outloud. I was thinking about doing something similar.
I had tried to build a DC/DC charger that had some charge profile. The renology wanderer (10A PWM, 2 usb outs, and a digital display) was configurable from 12V to 16V. In my tests however, when I hooked up the unit to another battery, recognized the input (14.4v) and the state of the battery (12.5v) but did not deliver any power to it. I will have to increase the voltage on the input side to see when the charging happens, because its not listed anywhere.
In my specific use case, the rear accessory outlet is always on. However, I would like to charge my LFP battery which is hooked up to my portable fridge, without tying it directly in as it would then use the LFP + starter battery as a singular unit. I have been plugging/unplugging the charger "as needed" but it sucks to have to do that constantly.

The reason I like your setup a lot is because of the voltage loss, the vehicle reads around 12.5V at the rear outlet with the car off, and anywhere from 13.3-14.5 with the car on. So the converter would be good for that, as it would self cut off once the engine was off. For longevity as well, I like the the fact that I can cut the target battery charge voltage, so I can target, say a 13V top, but it would only charge when the car is on.
 
You have about a 1kw pack? I guess 400W could charge that in one day. Sorry for thinking outloud. I was thinking about doing something similar.
I had tried to build a DC/DC charger that had some charge profile. The renology wanderer (10A PWM, 2 usb outs, and a digital display) was configurable from 12V to 16V. In my tests however, when I hooked up the unit to another battery, recognized the input (14.4v) and the state of the battery (12.5v) but did not deliver any power to it. I will have to increase the voltage on the input side to see when the charging happens, because its not listed anywhere.
In my specific use case, the rear accessory outlet is always on. However, I would like to charge my LFP battery which is hooked up to my portable fridge, without tying it directly in as it would then use the LFP + starter battery as a singular unit. I have been plugging/unplugging the charger "as needed" but it sucks to have to do that constantly.

The reason I like your setup a lot is because of the voltage loss, the vehicle reads around 12.5V at the rear outlet with the car off, and anywhere from 13.3-14.5 with the car on. So the converter would be good for that, as it would self cut off once the engine was off. For longevity as well, I like the the fact that I can cut the target battery charge voltage, so I can target, say a 13V top, but it would only charge when the car is on.
That sounds like a good fit to me. Yes, the battery that I tested is rated at 40 amp hour, 24v, so about 1KWH. I have a Li-Ion 24v that is about 100ah, which I have not tested with this setup yet. I also have cells to build a 60 amp hour LiFePo4 to replace the 40 amp hour with. I say "replace" because the toolbox that I have the 40 amp hour in (inverter in lid) has a lot more room. I had built the 40 amp hour for another box but stuck it in this box when I wanted to add the inverter to the solution.

On our deer camp site, I just had a couple of additional panels become available. Maybe I will have more like 800w PV available. I just have to decide if I want to mount all 4 or just stay with the 2 panels.
 
You have about a 1kw pack? I guess 400W could charge that in one day. Sorry for thinking outloud. I was thinking about doing something similar.
I had tried to build a DC/DC charger that had some charge profile. The renology wanderer (10A PWM, 2 usb outs, and a digital display) was configurable from 12V to 16V. In my tests however, when I hooked up the unit to another battery, recognized the input (14.4v) and the state of the battery (12.5v) but did not deliver any power to it. I will have to increase the voltage on the input side to see when the charging happens, because its not listed anywhere.
In my specific use case, the rear accessory outlet is always on. However, I would like to charge my LFP battery which is hooked up to my portable fridge, without tying it directly in as it would then use the LFP + starter battery as a singular unit. I have been plugging/unplugging the charger "as needed" but it sucks to have to do that constantly.

The reason I like your setup a lot is because of the voltage loss, the vehicle reads around 12.5V at the rear outlet with the car off, and anywhere from 13.3-14.5 with the car on. So the converter would be good for that, as it would self cut off once the engine was off. For longevity as well, I like the the fact that I can cut the target battery charge voltage, so I can target, say a 13V top, but it would only charge when the car is on.
Note, often a boost or a buck converter wants to see a voltage difference from input to output. It is hard to boost from 14.0v to 14.4v, for example. Some boards may work better than others do.
 
Here's a fun one. I've been using a Goal Zero Guardian- with a Yeti DC to DC charger for over 5 years. Maybe 7...
It has a cigarette lighter, to a power brick. The power brick goes into the solar feed port of the Guardian. And the guardian charges the battery.

Its worked pretty much flawlessly. It will even work when the car is below 12V. So it must be a boost to PWM charge.

My original goal was to do something similar; dedicated voltage cut off device, which cuts power. A boost, to feed the solar in port of my renology wander. Then let the solar charge controller charge the battery.

I am now thinking there isn't too many ways around this, which is basically I need a boost and then a buck.
I don't need something as heavy duty as yours, as my wiring is limited in carrying capacity, so I would probably just keep charging rates like 5-8amps to keep the strain on the alternator to something reasonable.
 
We took the golf cart to deer camp. I put up some used panels, two 60 cell and two 72 cell in 2s2p such that one 60 cell and one 72 cell are in series. They have very close to the same amp rating, so work fine.

We are in the middle of some tall trees. Some were cut in 2020 to give light to a 250w panel that we have to charge batteries for lighting. The high end of this (image below) array is setting on two large stumps and the front on some other wood. I hope to get back in Nov and secure them or put 4x4 posts under the array. We got about 2.5 hours of good sun per day and were able to keep both the 24v buffer battery and the cart battery topped off almost every day.

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Another photo showing charger setup. 50ft 14ga extension cord brings 70v PV from array to side of camp road. EPEver AN 40 amp delivers 24v output. 24v to 36v converter is set to deliver 10amps on the 36v side, so will pull about 360-400 watts from the battery/charger.


1666182933584.png
 
Another photo showing charger setup. 50ft 14ga extension cord brings 70v PV from array to side of camp road. EPEver AN 40 amp delivers 24v output. 24v to 36v converter is set to deliver 10amps on the 36v side, so will pull about 360-400 watts from the battery/charger.


View attachment 117005
So you are connecting the PV array directly to an Epever SCC programmed to deliver 24V charge current and then the boost converter is boosting output voltage from 24V (or 28.8V or whatever) up to 36V (or 43.2V or whatever)?

I’m very interested in this approach and may attempt something similar to charge a 48V LiFePO4 battery from a ~35VDC solar panel but one thing I do not understand is the behavior of the booster when input power is insufficient to deliver full programmed output current at full programmed output voltage.

Boost converter is set to deliver 10A @ 36V = 360W.

Epever SCC is set to deliver incoming solar power @ 24VDC.

So when incoming solar power is only 180W, Epever is putting out 7.5A @ 24VDC = 180W (assuming 100% efficiency).

The boost converter wants to pump that 24V up to 36V but when it does so, it cannot generate a full 10A but only ~5A (or 4.5A if we assume 90% efficiency).

So is that what the boost converter does when it does not receive sufficient inout power to deliver full output current at full output voltage?

Whatever solar power is available is converted to whatever current can be delivered at full boost voltage?

Or when underpowered does the boost converter attempt to maintain full output current even if that means lower output voltage? (10A @ 18VDC or 16.2VDC in this example).
 
More designs from the Amateur Electrical League ?
I believe I’ve answered my own question - boost converters convert incoming power to the maximum current they can deliver at specified voltage.

Pretty much every MPPT SCC out there is buck (incoming PV voltage must be higher than battery voltage).

But we’re starting to see more and more boost MPPT chargers: https://www.amazon.com/Renogy-Contr...cphy=9032079&hvtargid=pla-1206462025871&psc=1

Or https://www.amazon.com/Solar-Charge...mzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc

Or https://www.amazon.com/Controller-A...D5&keywords=solar+boost+charger&qid=169040531 &sprefix=solar+boost+charger%2Caps%2C145&sr=8-9-spons&ufe=app_do%3Aamzn1.fos.006c50ae-5d4c-4777-9bc0-4513d670b6bc&sp_csd=d2lkZ2V0TmFtZT1zcF9waG9uZV9zZWFyY2hfbXRm&psc=1

Or https://www.amazon.com/ECO-WORTHY-C...9&sprefix=solar+boost+charger,aps,145&sr=8-12

My fallback plan will be to pick up one of these boost SCCs but since I already have a 60A Epever SCC and an 1800W DCDC booster, I’ll probably give that a try that first.

The Epever SCC will maximize solar output with it’s MPPT and the maximum output voltage I can program it for is 34VDC.

The DCDC booster will boost that 34VDC to 57.6V (169%).

Most of them seem to be limited to 10A of charge current or 600W, so it may be some time before we see higher-powered boost SCCs, but for a parallel string of 1-3 panels look like charging a higher-voltage battery is pretty doable…
 
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