| Check my electrical math- battery to inverter to Air Con [message #370334] |
Wed, 10 August 2022 08:46  |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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THis is actually for a sprinter van but the kids on those forums are sometime difficult and un-civil
An 8000 BTU window AC unit will be split in two so the condenser is outside the vehicle (very cool idea from a van guy)
AC draws 800 watts - that's 66 amps at 12V (round up to 70 amps for the inverter)
Duracell GC2 6V battery will provide 75 amps for 120 min.
All flooded battery numbers are cut in half for real world usage so two of these batteries in series for 12V should provide 75 amps for one hour, right?
But the 25 amp discharge rate is 448 minutes so if I half that (224 min) I can get almost four hours with three pairs of batteries.
Do i have this right?
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: Check my electrical math- battery to inverter to Air Con [message #370335 is a reply to message #370334] |
Wed, 10 August 2022 09:29  |
kingd
Messages: 592
Registered: June 2004
Karma: 2
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Dave, remember the available "amp-hours" are at the "rated" current draw. I believe if the actual draw is a lot higher
than the "rated" draw you won't get any where near the "rated" amp-hours.
See if you can find what the real world amp-hours is at 75 amps. That's a lot of amps.
DAVE KING
lurker, wannabe
Toronto, Ontario, Canada
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| Re: Check my electrical math- battery to inverter to Air Con [message #370337 is a reply to message #370335] |
Wed, 10 August 2022 09:52 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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kingd wrote on Wed, 10 August 2022 09:29Dave, remember the available "amp-hours" are at the "rated" current draw. I believe if the actual draw is a lot higher
than the "rated" draw you won't get any where near the "rated" amp-hours.
See if you can find what the real world amp-hours is at 75 amps. That's a lot of amps.
Exactly- I think I'm accounting for that.
The manufacturer says the battery will deliver 75 amps for 120 min. but full discharge shortens the life of the battery so we go 60 min.
But that's for one pair of 6v batteries.
If we have three pairs then we have 25 amps per battery pair for 440 min- or half that for 220 min. or nealry 4 hours for $700
I also looked at some golf cart data- they are typically pulling 30 amps per battery to cruise at top speed so that seems like a sweet spot for the battery life cycle.
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: Check my electrical math- battery to inverter to Air Con [message #370339 is a reply to message #370334] |
Wed, 10 August 2022 10:08 |
hertfordnc
Messages: 1164
Registered: September 2009
Location: East NC
Karma: 0
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It turns out this might all be moot- if my math is right i can get three hours of discharge at 70 amps if i use SIX Duracell GC batteries @ about $120 each.
For $700 I can get a 200 amp Lithium battery and discharge it at 70 amps for the same three hours.
I think we've crossed that line.
Dave & Ellen Silva
Hertford, NC
76 Birchaven, 1-ton and other stuff
Currently planning the Great american Road Trip Summer 2021
It's gonna take a lot of Adderall to get this thing right.
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| Re: Check my electrical math- battery to inverter to Air Con [message #370342 is a reply to message #370339] |
Wed, 10 August 2022 20:46 |
Melbo
Messages: 144
Registered: August 2018
Location: Albuquerque NM
Karma: 0
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Long story short.
I did exactly that -- split a window ac and powered it with a 4024 inverter. I had 6 8D absorbed glass mat batteries. The math said I could run it all night and charge the batteries driving during the day. About 2-4 hours in I had to get up and fire up the generator. The math works on the paper but not in real life. I did however learn a great deal of what is necessary to be comforable when traveling.
I hope this helps.
The Melbo
Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
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| Re: Check my electrical math- battery to inverter to Air Con [message #370344 is a reply to message #370343] |
Wed, 10 August 2022 23:04 |
Melbo
Messages: 144
Registered: August 2018
Location: Albuquerque NM
Karma: 0
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I have been curious about splitting an inverter type window unit…do you have any pics or links that might show what you did? Did you add distance or just remove the shell to allow for a wall instead of a window?
I did this in my bus conversion. I did it 15 years ago. I bought two window type ac units cut them up and put the condenser part in the engine bay and the evap and fan under the bed with the air blowing into the sleeping area. I ran the tubing through the engine bulk head and sealed it well.
I was one of my many failed experiments when building a bus conversion. I now know the benefit of a good and reliable inverter engine alternator and reliable generator.
The Melbo
Albuquerque NM Bus Conversion 1978 MCI 1973 GMC
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| Re: Check my electrical math- battery to inverter to Air Con [message #370345 is a reply to message #370334] |
Thu, 11 August 2022 08:07 |
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RF_Burns
Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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"It turns out this might all be moot- if my math is right i can get three hours of discharge at 70 amps if i use SIX Duracell GC batteries @ about $120 each.
For $700 I can get a 200 amp Lithium battery and discharge it at 70 amps for the same three hours.
I think we've crossed that line."
Dave,
You will find Lithium (LiFePO4 aka LFP) are a much better battery than Lead Acid (LA). You can charge them in a much shorter time, they give back almost as much energy as you put into them (read efficient) and their discharge curve is flatter and at a higher voltage, not to mention they are much lighter in weight.
You will love them!
A $700, 200 Ahr LFP might not be the highest quality on the market though, you get what you pay for. While you can (should) get all 200ahr out of the battery, you will get more charge cycles if you limit the depth of discharge (DOD) to something less 100%. Aiming for 80% DOD might be more realistic for the battery price point you are looking at.
You will also need a beefy LFP rated charger to recharge it, both from shore power and while driving. A 60A charger is going to take just over 3 hours to fully recharge a flat 200Ahr battery and unlike a LA charge cycle, a LFP battery will consume that 60A the whole time. While driving, I recommend a DC-DC charger to recharge from the vehicle alternator. These chargers take the vehicle power and transform it to the proper LFP charging curve and properly limit the current. On the other hand, a BIM combiner directly connects the alternator to the LFP battery for 15 minutes then disconnects for 20 minutes to let the alternator cool down (like a very slow cycle PWM regulator). While connected, the LFP battery will consume all the current the alternator can give and likely more than its rated for.
You might want to check out Will Prowse's solar forum for some good info on LFP batteries and charging. Also check his many videos on youtube.
https://diysolarforum.com/
Here is an excellent article on LFP batteries. It is written for boaters, but most info also covers RV applications.
https://marinehowto.com/lifepo4-batteries-on-boats
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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| Re: Check my electrical math- battery to inverter to Air Con [message #370348 is a reply to message #370334] |
Thu, 11 August 2022 09:03 |
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RF_Burns
Messages: 2276
Registered: June 2008
Location: S. Ontario, Canada
Karma: 3
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Larry,
The main downside of LFP batteries is cold weather charging. You can store and discharge the battery below freezing (0C), but you must not recharge them when the battery temperature is below zero as it will damage them. Most Battery Management Systems (BMS) will prevent charging when the battery temperature is below freezing.
There are cold weather rated LFP batteries. These batteries will use available charging current to power an internal heating pad to raise the temperature of the battery to above freezing first, then the BMS will allow the battery to be charged.
Since a LFP battery likes the temperatures we human like (not too hot and not too cold) and they are safe, many LFP batteries are installed inside the coach. Mine are in the bottom of the closet, right beside the electrical cabinet.
Here is a cold weather rated LFP battery with Bluetooth monitoring.
https://www.ablithium.ca/products#!/PRE-ORDER-12V-100AH-Extreme-Series-Heated-Bluetooth-LiFePo4-Battery-Expected-in-Warehouse-August-25th
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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