Running a microwave in a hunting cabin

[Delaware79]

Hi there all.
I have recently dipped in to solar so we could have peaceful nights not hearing the generator run. We only use the cabin once a year for 7 days. what size battery bank could support using a small microwave 800w and not kill the battery’s due to heavy load. And what kind of batteries can handle this? I’m thinking a 400ah bank with lithium. Thanks in advance.

 

[John Frum]

for lead acid batteries $dod = .5
for lifepo4 battereis $dod = .8

to calculate your usage
microwave watts * hours = ac watt hours
ac watt hours / .85 inverter conversion efficiency = dc watt hours
dc watt hours * $dod depth of discharge = usable dc watt hours
usable dc watt hours / battery nominal voltage = battery amp hours

IMO choose lifepo4 unless there is a compelling reason not to.
For example charging below 0 celcius will damage lifepo4 batteries.

 

[gnubie]

The question is how long will the oven be running and what voltage is your inverter's input? Running the oven for 15 minutes is only about 300 watt hours (800 watts of microwaves uses about 1200 watts of power into the oven). That's nothing in terms of battery capacity.

What about amps? Let's say its a 12v inverter. That's around 125 amps (assume lead acid sag to 11.5v and inverter efficiency). For a 24v inverter that becomes about 63 amps. Let's be kind to the battery and assume discharge at 0.5c. 0.5c means we need 250AH 12v battery or a 125AH 24v battery. Of course that's being kind. You don't have to be you could run them hard so 125AH and 63AH respectively.

OK, so what about lithium.
Lithium has no problem holding the voltage up unlike its tired old cousin, lead acid. 12v inverter, about 115 amps (battery at 12.5v). At 24v that becomes about 57 amps (battery at 25v). At 0.5c 230AH and 114AH respectively, but a respectable lithium has no problems with 1C discharge so 115AH and 57AH.

2 x battleborn 12v 100AH battery will do the trick. Obviously there are cheaper lithium options too and for that matter lithium batteries that are quite happy with discharge rates over 1C. My own are rated at 1.4C continuous.

With this sort of low intermittent use AGM batteries are going to run into electrolyte loss issues just by being parked on a float charge. If you plan on keeping the cabin visits up in the long term you are going to run into this problem with the AGMs well before any charge / discharge cycle issues.

You have to think about your inverter too. A cheapie HF inverter at 12v may have trouble running a standard heavy transformer microwave. It wouldn't be the first time that someone has reported this on the forum. If you have an inverter based oven it should be OK even with a cheapie inverter.

A 400 AH lithium battery could do it standing on one foot with one arm tied behind its back.

*edit, left inverter losses out of lithium*

 

[Delaware79]

The question is how long will the oven be running and what voltage is your inverter's input? Running the oven for 15 minutes is only about 300 watt hours (800 watts of microwaves uses about 1200 watts of power into the oven). That's nothing in terms of battery capacity.

What about amps? Let's say its a 12v inverter. That's around 125 amps (assume lead acid sag to 11.5v and inverter efficiency). For a 24v inverter that becomes about 63 amps. Let's be kind to the battery and assume discharge at 0.5c. 0.5c means we need 250AH 12v battery or a 125AH 24v battery. Of course that's being kind. You don't have to be you could run them hard so 125AH and 63AH respectively.

OK, so what about lithium.
Lithium has no problem holding the voltage up unlike its tired old cousin, lead acid. 12v inverter, about 115 amps (battery at 12.5v). At 24v that becomes about 57 amps (battery at 25v). At 0.5c 230AH and 114AH respectively, but a respectable lithium has no problems with 1C discharge so 115AH and 57AH.

2 x battleborn 12v 100AH battery will do the trick. Obviously there are cheaper lithium options too and for that matter lithium batteries that are quite happy with discharge rates over 1C. My own are rated at 1.4C continuous.

With this sort of low intermittent use AGM batteries are going to run into electrolyte loss issues just by being parked on a float charge. If you plan on keeping the cabin visits up in the long term you are going to run into this problem with the AGMs well before any charge / discharge cycle issues.

You have to think about your inverter too. A cheapie HF inverter at 12v may have trouble running a standard heavy transformer microwave. It wouldn't be the first time that someone has reported this on the forum. If you have an inverter based oven it should be OK even with a cheapie inverter.

A 400 AH lithium battery could do it standing on one foot with one arm tied behind its back.

*edit, left inverter losses out of lithium*

This is my system but looking at lithium for down the road or if I’m dissatisfied with the batteries.
2x 6v NPP NP6-180ah
Epever 30a mppt
Inverter wzrelb reliable 3000w/6000w
Newpowa 175w monocrystalline panel.
this is a 12v I put for inverter.
The microwave would be used 2-6 min at a time twice a day.
I now believe the 6v set up may not do the job but knowing nothing about solar to what I think I know nowfrom Wills videos,I think I’m doing pretty good. I plan on seeing how long the panel will keep it going and ok with just going to generator to get us through the week if need be, then add to the system. Any advice to this set up is thanked! I hope this detail helps you all help me.

 

[Delaware79]

This is my system but looking at lithium for down the road or if I’m dissatisfied with the batteries.
2x 6v NPP NP6-180ah
Epever 30a mppt
Inverter wzrelb reliable 3000w/6000w
Newpowa 175w monocrystalline panel.
this is a 12v I put for inverter.
The microwave would be used 2-6 min at a time twice a day.
I now believe the 6v set up may not do the job but knowing nothing about solar to what I think I know nowfrom Wills videos,I think I’m doing pretty good. I plan on seeing how long the panel will keep it going and ok with just going to generator to get us through the week if need be, then add to the system. Any advice to this set up is thanked! I hope this detail helps you all help me.

Forgot to state the batteries are agm not flooded

 

[gnubie]

OK, so not your regular run of the mill AGM. These have a design life of 10 years at float.

At new, running the oven would be about 0.7C so not terrible. The spec sheet doesn't detail the effects of higher discharge rates are on the life of the battery only that it can be done and the amount of time the battery will hold the load at those rates. At 0.7C the new battery (2 6v in series) will sag to around 12v in the time the oven is being run so the inverter should be happy with a resistive load. With a big transformer oven it could be in trouble still.

The DOD with ~15 mins of oven run time with a new battery is about 14%. At that DOD you can expect somewhere around 1200 cycles with the battery ending its life around 70% initial capacity at 0.5c.

It'll work, and work for several years given the low use but exactly how many years is a good guess. Unfortunately a lot of this is guess work as the all important discharge rate vs cycle lifetime isn't mentioned in the specs.

A single 175 watt panel should be able to replace the energy used by the oven in 2.5 hours of square on sunlight and from there it'll be lifting the battery to replace losses and keeping it at float. Check the expected insolation at your location (check the FAQ for a map) and don't forget the effects of weather. I'd recommend you get a 24v class panel rather than a 12v one when using a MPPT controller.

 

[Delaware79]

OK, so not your regular run of the mill AGM. These have a design life of 10 years at float.

At new, running the oven would be about 0.7C so not terrible. The spec sheet doesn't detail the effects of higher discharge rates are on the life of the battery only that it can be done and the amount of time the battery will hold the load at those rates. At 0.7C the new battery (2 6v in series) will sag to around 12v in the time the oven is being run so the inverter should be happy with a resistive load. With a big transformer oven it could be in trouble still.

The DOD with ~15 mins of oven run time with a new battery is about 14%. At that DOD you can expect somewhere around 1200 cycles with the battery ending its life around 70% initial capacity at 0.5c.

It'll work, and work for several years given the low use but exactly how many years is a good guess. Unfortunately a lot of this is guess work as the all important discharge rate vs cycle lifetime isn't mentioned in the specs.

A single 175 watt panel should be able to replace the energy used by the oven in 2.5 hours of square on sunlight and from there it'll be lifting the battery to replace losses and keeping it at float. Check the expected insolation at your location (check the FAQ for a map) and don't forget the effects of weather. I'd recommend you get a 24v class panel rather than a 12v one when using a MPPT controller.

Yes the batteries I thought were middle of the road for the purpose. The whole system I think is middle of the road and I didn’t want to go cheap since you get what you pay for, but I couldn’t see building a $2000 system for some lights and possibly a microwave for 1 week out of the year. As for the 24v panels you recommend, I never looked at them since I thought the whole system had to be 12v. Can I use a 24v panel to a 12v inverter? I’ll be researching that next. What I did do was I got the panel cables at 8gauge to hopefully help getting better charge. Why is 24v panel work better for mppt? I’ll have to research that too. I’ve done soooo much research in the past 4 months on this, and finally reached out to this forum because of Will’s Utube videos were so very helpful. I cannot thank y’all enough!

 

[gnubie]

Yes, you can use a 24v class panel. The MPPT controller will take care of changing the panel volts to the battery voltage without much power loss, and the inverter only has to be spec'd for battery voltage. If you already have a 12v panel, that's OK it will work but MPPT controllers like a bit of headroom to work with.

 

[PHoganDive]

Another thing to consider is theft, if the place is left unattended, especially for long lengths of time. My experience with cabins is that anything of any value tends to disappear by the next time you visit, so you have to take it with you.

 

[Delaware79]

Another thing to consider is theft, if the place is left unattended, especially for long lengths of time. My experience with cabins is that anything of any value tends to disappear by the next time you visit, so you have to take it with you.

You are 100% correct! We take all valuables back home with us due to that reason. I would have to take the batteries regardless since they would drain and go bad throughout the year not being used. It’s a pain transporting all our stuff but it’s a safeguard.

 

[Delaware79]

OK, so not your regular run of the mill AGM. These have a design life of 10 years at float.

At new, running the oven would be about 0.7C so not terrible. The spec sheet doesn't detail the effects of higher discharge rates are on the life of the battery only that it can be done and the amount of time the battery will hold the load at those rates. At 0.7C the new battery (2 6v in series) will sag to around 12v in the time the oven is being run so the inverter should be happy with a resistive load. With a big transformer oven it could be in trouble still.

The DOD with ~15 mins of oven run time with a new battery is about 14%. At that DOD you can expect somewhere around 1200 cycles with the battery ending its life around 70% initial capacity at 0.5c.

It'll work, and work for several years given the low use but exactly how many years is a good guess. Unfortunately a lot of this is guess work as the all important discharge rate vs cycle lifetime isn't mentioned in the specs.

A single 175 watt panel should be able to replace the energy used by the oven in 2.5 hours of square on sunlight and from there it'll be lifting the battery to replace losses and keeping it at float. Check the expected insolation at your location (check the FAQ for a map) and don't forget the effects of weather. I'd recommend you get a 24v class panel rather than a 12v one when using a MPPT controller.

Yes the batteries I thought were middle of the road for the purpose. The whole system I think is middle of the road and I didn’t want to go cheap since you get what you pay for, but I couldn’t see building a $2000 system for some lights and possibly a microwave for 1 week out of the year. As for the 24v panels you recommend, I never looked at them since I thought the whole system had to be 12v. Can I use a 24v panel to a 12v inverter? I’ll be researching that next. What I did do was I got the panel cables at 8gauge to hopefully help getting better charge. Why is 24v panel work better for mppt? I’ll have to research that too. I’ve done soooo much research in the past 4 months on this, and finally reached out to this forum because of Will’s Utube videos were so very helpful. I cannot thank y’all enough!

These are some pics of the battery specs since the batteries just arrived. Also my charge controller has not arrived, but I have read somewhere that I do not have to program the charge rate for my agm batteries. Is this true? I will see how well things work out this year with the 12v panel but now after researching, I agree I should have gotten 24v panel.

 

[gnubie]

Odds on the epever will arrive with SLA as the setting, but it wouldn't hurt to check and read the manual too to make sure SLA doesn't do periodic equalisation. If it does, IMO, you'll want to turn that off for AGM batteries. It will auto-detect the battery voltage by default too.

 

[Brewman]

Mine did and the instruction manual says it's sealed as default

 

[Delaware79]

Odds on the epever will arrive with SLA as the setting, but it wouldn't hurt to check and read the manual too to make sure SLA doesn't do periodic equalisation. If it does, IMO, you'll want to turn that off for AGM batteries. It will auto-detect the battery voltage by default too.

Great info from you, thanks again!

 

[Delaware79]

Mine did and the instruction manual says it's sealed as default

Awesome thanks for verifying!