# 18vdc to 12.6vdc using a pwm to run a 12cdv 600 watt inverter

#### roy michael

Joined Aug 14, 2013
4
Will the pwm run the inverter or will spikes damage it

#### geekoftheweek

Joined Oct 6, 2013
529
If it is all designed right the answers will be yes and no. If it is not then you will end up with maybe and yes. It's going to take a lot more information. I just clicked on here in hopes of learning something. Good luck

#### BobTPH

Joined Jun 5, 2013
2,541
Disagree. Raw PWM will run a heater or incandescent bulb as is it were running from a constant voltage at the input x the duty cycle since the load is purely resistive and the total power delivered is all that is important. It will also run a motor because it is inductive.

An inverter is an electronic circuit that would require a constant voltage to run correctly.

You would need either an inverter designed for 18V or a buck converter in between.

Bob

#### roy michael

Joined Aug 14, 2013
4
If it is all designed right the answers will be yes and no. If it is not then you will end up with maybe and yes. It's going to take a lot more information. I just clicked on here in hopes of learning something. Good luck
I have 2 6v batts in series, but to extend the run time, I would like to use 3 batts in series for 18v . Using off grid to run a small fridge 120vac .087 amps
Disagree. Raw PWM will run a heater or incandescent bulb as is it were running from a constant voltage at the input x the duty cycle since the load is purely resistive and the total power delivered is all that is important. It will also run a motor because it is inductive.

An inverter is an electronic circuit that would require a constant voltage to run correctly.

You would need either an inverter designed for 18V or a buck converter in between.

Bob
Can I smooth out the pwm voltage

#### BobTPH

Joined Jun 5, 2013
2,541
Yes, that is what a buck converter does. You would be better off to get two more batteries and make a series parallel 12V battery.

Bob

#### roy michael

Joined Aug 14, 2013
4
Yes, that is what a buck converter does. You would be better off to get two more batteries and make a series parallel 12V battery.

Bob
Ok, here's what I have, 6 6v batts, 3 60w solar panels, batts 220ah. At the present i have 3 sets of 12, each on 1 solar panel. I rotate a set each time I go to the cabin, every week or two, that way, I have fresh batts each time. It seem that I don't. Have enough sun for the weekend I their.
Yes, that is what a buck converter does. You would be better off to get two more batteries and make a series parallel 12V battery.

Bob

#### roy michael

Joined Aug 14, 2013
4
Ok, here's what I have, 6 6v batts, 3 60w solar panels, batts 220ah. At the present i have 3 sets of 12, each on 1 solar panel. I rotate a set each time I go to the cabin, every week or two, that way, I have fresh batts each time. It seem that I don't. Have enough sun for the weekend I their.
Oh by the way. Thank you for helping me. I'm 71 years old and just playing around try to stay off grid for fun, know little about electronics. The only thing I want is to run a very small fridge only from converter. Thanks

#### Tonyr1084

Joined Sep 24, 2015
5,080
Will the pwm run the inverter or will spikes damage it
PWM is what Buck Converters run on. Well, not necessarily PWM, but they DO change DC into AC, boost or drop the voltage, then convert back to DC. However, overall wattage remains the same. 18 volts at 1 amp is 18 watts. Convert that down to 12 volts and you get 1.5 amps. 12 volts times 1.5 amps equals 18 watts. And THAT is in a PERFECT world. Anytime you convert you give up something due to inefficiencies. Like BobTPH said - you're better off increasing the capacity of the power supply rather than increasing the voltage. Higher voltage means more heat lost due to those inefficiencies mentioned.

Can I smooth out the pwm voltage
Yes. Buck converters do a fairly good job of it on their own, but you CAN enhance the smoothing with a large capacitor. But there again you run into inefficiency losses. The more you condition a signal the more you give up. There's a trade-off between what you give up and what you achieve.

It's my belief the best approach is - since you say you have so many six volt batteries - is to configure them into 12 volt banks. Then put them in parallel. Since they're charged via solar PV panels (Photo Voltaic) there would be no reason why you couldn't keep them all on the charger all the time. Just make sure the batteries are properly protected from over-voltage.

Another thing to keep in mind with batteries - in particular, Lead Acid or Sealed Lead Acid batteries is that they should be kept charged. If allowed to go long periods of time without being charged they can develop sulfates on the battery plates, which will limit the amount of current the battery can produce. You may have 100 Ah's by the numbers, but your actual available power will be reduced, dependent on how much sulfating has occurred. If the battery is 50% sulfated then you'll only have 50 Ah's of power.

Finally, if you're spending money - why not get a fridge that operates on 12 volts DC. They have them. I don't know how expensive they are, but you don't have losses from an inverter. Good inverters can be fairly efficient, but still you could be losing up to 20% of your available power. Cheap ones can be down around 60% efficient, losing 40% to heat and other inefficiencies in the design. Also, those cheaper ones don't produce as clean a signal as the more expensive ones. So take a grain of salt along with your plans for a fridge off the grid.

#### crutschow

Joined Mar 14, 2008
25,432
I agree that you should stay with 12V as you don't want the energy loss of converting from 18V to 12V.

You might also consider adding another solar panel.

#### BobTPH

Joined Jun 5, 2013
2,541
What is the reasoning behind having three batteries with two remaining idle all of the time? You could simply put all of them in parallel.

Bob

#### Tonyr1084

Joined Sep 24, 2015
5,080
have, 6 6v batts, 3 60w solar panels, batts 220ah. At the present i have 3 sets of 12, each on 1 solar panel. I rotate a set each time I go to the cabin, every week or two, that way, I have fresh batts each time.
OK, you have SIX 6V 220Ah batteries and you're setting them up in sets of THREE batteries? Each on one solar panel? Are you saying you have three PV Panels? Why not combine them all. Take three sets of two batteries (two six volt batteries in series) (make three sets of them) Connect the sets in parallel. Use all three PV Panels in parallel to charge the batteries all the time. Then you have as much power as you're capable of capturing. No need for switching out batteries every time you go. If configured the way suggested you can have 660 Ah available. Depending on how heavily you use them you'll either make it through the weekend or possibly not. Using an inverter is going to waste some of that available power.

Instead of an inverter - get a 12 volt fridge. They make them. Here is one now. $300 US #### CROSSBOLT Joined Dec 9, 2008 21 You want to extend run time with batteries you hook up 12 volts in PARALLEL. So, if you have a pile of 6 volt batteries, you use series in pairs to get 12 volts nominal THEN parallel the pairs to increase amp-hours. More amp-hours = more run time. Same with solar panels. #### Rich2 Joined Mar 3, 2014 172 OK, you have SIX 6V 220Ah batteries and you're setting them up in sets of THREE batteries? Each on one solar panel? Are you saying you have three PV Panels? Why not combine them all. Take three sets of two batteries (two six volt batteries in series) (make three sets of them) Connect the sets in parallel. Use all three PV Panels in parallel to charge the batteries all the time. Then you have as much power as you're capable of capturing. No need for switching out batteries every time you go. If configured the way suggested you can have 660 Ah available. Depending on how heavily you use them you'll either make it through the weekend or possibly not. Using an inverter is going to waste some of that available power. Instead of an inverter - get a 12 volt fridge. They make them. Here is one now.$300 US
That is an absorption fridge that will use 7.5amps at 12v
A compressor fridge is much more efficient, our 12v compressor fridge runs between 2 and 2.5amps depending on temperature.

#### MisterBill2

Joined Jan 23, 2018
7,031
I am also wondering why add another battery to the string of batteries. Use the inverter at the voltage it was designed for, and use a regulated switching power supply fed from the solar cells to charge the batteries. That will be more efficient and more reliable and also cost less.