Battery requirements

[Pokey]

I devour topics on batteries voraciously but I do find some of the figures used as being confusing and maybe a little on the light side. Common comments throughout the forum refer to 100Ah and sometimes up too 200Ah setups, this may be enough but If driving any sized fridge, for any length of time, I can see it being extremely marginal, no matter what battery is used or how perfectly it stores and releases energy. My basic electrical theory was picked up many years ago so I may be making an error here somewhere so please correct my error if I am.

From what I can pick up on the net any reasonable fridge, based on a 180 to 240L unit, is going to draw somewhere between 800 and 1000 W a day. Now most of my research has been in 240V fridges as I am convinced that they are most economical way to go for long term or full time travel. The best of these, according to the Governments energy rating web site, can expect to draw somewhere between 300 and 400KWh per year. To me that averages out at 821 to 1095 Wh per day which is what I base my figures. Now lets assume that you have the better of these and work at a perfect 12V, this would give 68.4Ah draw per day (821/12=68.4). 12V fridges I understand are going to work out somewhere similar. Now back to battery size, based on this daily draw of power you are going to need a minimum of 69Ah of batteries, again assuming perfect batteries with no loss, this of course is if you want to buy a new one every week or so as 100% drain will kill them and you will need to generate the same 69Ah each day to recharge it. So to keep the batteries alive for a while we need to look at max DOD. Lithium guys tell me they can get them down to 20% and still have good battery performance, I know you can draw an AGM down that far and still have it live but it's life expectancy will be severely depleted. For me currently running AGM's I look at 60% DOD so we need to have a minimum of 110Ah of batteries and as they are not perfect I now if you allow for loss in batteries, wiring, regulator, PV performance etc etc which varies from set up to set up but lets be generous and call it 15% so this pushes up the storage capacity too 130Ah, still within the aforementioned 200Ah setup but lets put another spanner in the works. You can't guarantee perfect weather for solar generation every day, it occasionally rains. So if you want to play your solar safely you need to allow a margin for days without any input. Now this figure is entirely up to the individual. If they are happy to run a generator a lot, 1 day is enough, a bit less and 2 days reserve is needed, very occasionally and it means 3 days storage, so with our above figures we are talking about 130Ah for 1 day, 260Ah for 2 days and 390Ah for 3 days. I don't allow for any generation from the vehicle as I am looking at getting a good look at the place and not driving every day and even every week. Now remember this is for a fridge only, no lights, no TV, no radio, no computer, nothing else. I know that this is working at 50% DOD and with lithium you will get away with less but it still means you need a minimum of 85, 170 and 255Ah respectively with a projected max DOD of 20%.

Now my calculations for my set up suggest much higher storage needs then this because it includes running other appliances like radios, TV, lights, water pumps, satellite TV accessories etc. It doesn't include things like washing machines which I would look at being used on good light days with the expectation of it to continue the next at least so it doesn't affect my emergency allowance for none generating days. My final setup will have 750W of panels on the roof and somewhere around 500Ah of batteries and I am still looking at it as being on the light side. If I have good bright days all the time I will be in overkill but I believe I need to allow for some not so good periods. As I said to start each set up is different but I base my calculations on the results put out by a spreadsheet I downloaded from Rich Electric and it seems to me to be a good guide taking into account all usage and allows for losses in storage, transmission and generation and I do recommend that you look for it and put your unit's figures into it when calculating battery storage and solar panel needs, whichever battery you use, I think it will surprise you.

[T1 Terry]

I agree to an extent, I don't think you have allowed enough for a 200ltr to 250ltr fridge, they will drag 1000Wh to 1200Wh per 24hrs when the weather warms up. Aim at the higher figure and it's 100Ah from a lead acid battery and 92Ah for the lithium. You need 2 days min storage, that's 184Ah for the lithium, a 200Ah lithium battery and you still have your 20% safety net but it's 400Ah for a lead acid battery with the 50% SOC limitation. With the fridge, you can't go much lower than 50% before the voltage is too low for the fridge to operate under load, 50% SOC under load drops to around 12v at the battery, 11.6v to 11.8v at the fridge, they aren't happy with that. A 240vac fridge powered via an inverter will fair better, the inverter will keep going down to 11v or so.
The lithium batteries will hold 13v under load down to around 20% SOC, 12v at 0% SOC, then it drops like a stone not long after but as soon as the load is gone they return to around 12.6v to 13v even though they have been completely drained, that's why a state of charge meter is required, reading voltage alone will have you caught short during poor solar times.

[TigerMK1]

To give an example of a fully working system. I have 8x90AH Lithium cells connected in a serial/parrallel setup to give us a 12V system. I also have a therotical 720W of solar. That translats to a max of 30-40A @13.5V on a good day. We have a 240V F&P 2 door domestic fridge running 24/7. We also have satellite TV, led lights, water pump, vacumn toilet, various fans and a diesel furnace. The usual stuff. Oh, I have also fitted a 620W electric element to our gas hot water system.
The fridge and all the AC appliances such as the TV,microwave and toaster are fed via a 2000W MSW inverter (Itried a PSW and it failed)

Now on a good sunny day I either turn off the solar at around 10am (the solar is controlled by a PL20 and a relay with a shunt) or I turn on the heater element in the hot water system.

The PL20 shows a typical 24hr draw from the system (fridge 24hr, inverter same, TV few hours, rest minor) at 85 to 96AH. The battery shows 13.4V when I go to bed and 13.1V when I wake up.

Now I do not recommend lithium to you, find out for yourself and be amazed.

[PandJ R away]

I will copy and support Tigers comments.

We run 200ah Li battery with our main draw being a 2 door Waeco. My consumption ranges form 85-128 ah per day depending upon the weather, use of the birko etc.

I have approx 600w P.V. panels which maintain the battery's at full charge at sundown.

The Li battery's replaced a 400ah AGM set and provides improved comfort outcomes ie boost to hot water calorifier, 240v air compressor, wood lathe etc

Ours consistently shows 13.4 at sunset and usually 13.2-3 at breakfast. The regulator shows 13.9v by 10.00am (or earlier) usually.

I made provision for an additional 200ah Li battery pack but I am not sure what I would do with the power and so I have delayed/postponed their purchase.,

In unexpected circumstances such as repair of the motorhome I reduce the temp setting on the fridge and don't open it as much.

John

[Craig]

TigerMK1,

Those figures obviously don't reflect operating the a/c from your lithium batteries, with that in mind you could get away with a system half that size and only AGM's. I would be interested in knowing how many AH's are consumed when running the A/C for the day? Also what fridge model do you have?

regards
Craig

[TigerMK1]

TigerMK1,

Those figures obviously don't reflect operating the a/c from your lithium batteries, with that in mind you could get away with a system half that size and only AGM's. I would be interested in knowing how many AH's are consumed when running the A/C for the day? Also what fridge model do you have?

regards
Craig

Hi Craig, the Fridge is a F&P 249T that we reseached as a good poqwer user and the right size for us. Yes you would go larger battery capacity if running a aircon for any extended time. We make do with the very efficient dyson blade less tower units and no aircon.

Regards
Steven

[T1 Terry]

TigerMK1,

Those figures obviously don't reflect operating the a/c from your lithium batteries, with that in mind you could get away with a system half that size and only AGM's. I would be interested in knowing how many AH's are consumed when running the A/C for the day? Also what fridge model do you have?

regards
Craig

Ok, a system half that size would be 90Ah and 360W solar. If they were AGM batteries there would be 45Ah available down to 12v, a realistic cut off voltage as most appliances besides inverters won't function correctly below that voltage, but for the sake of a comparison test we will use an inver and a cut off voltage of 11v so we will be taking a lot more than the 50% capacity.
The 2000w at full power would draw 2000w plus 10% from the battery, that's 200amps @ 11v, I doubt you could pull 200 amps from a 90Ah AGM and still see 11v but 11v is as low as the inverter will still operate so it's theoretical figure really. 200Amps from a 90Ah battery is a 2.2C load, a quick look on the Fullriver site to see how many Ah we can have at a 2.2C load thanks to Mr Peukert

Fullriver DC105ah AGM.JPG

This is a chart for a 105Ah Fullriver AGM so the first line represents a 3C load, the second line a 1C load so our 2.2C load is somewhere between the first 2 lines. The 3C load crosses the 11v line at roughly 2 mins, the 1C line at around 20 mins so say 10 mins @ 2.2C from a fully charged battery and it's empty. I don't think the AGM is up to the task.
Sure the inverter isn't going to run at full power for 10 mins but each time the microwave is used it will be close to that figure, for the first few secs when the fridge starts up it will be close to that figure.

What isn't being read into Steven's 85Ah to 96Ah draw recorded on the PL20 is all the solar that comes in after 10am, the point Steven says the batteries are fully recharged, maybe another 4 hrs @ 30amps to 40amps is still available? Useful charging ends around an hr before sun down so maybe 10 amps is available right up till 6pm, it's not till after that time the batteries are back on the job really, they carry the peak loads but there is enough solar charging available to quickly replace that and still supply the system load requirement because lithium batteries will simply recharge that quick, AGM batteries wouldn't return to 100% recharged in a whole day of solar with no loads at all, every time they have to supply a peak load it takes hrs to get that back into the battery, there is a huge difference between the two battery types.

T1 Terry

[Craig]

Why is it every time I ask a question about Lithium batteries I get a defensive answer and no one will answer the question I asked. I already now that AGM's will not support the types of loads that the lithiums will. I stated that in an earlier post so you don't need to prove that to me.

I would be interested in knowing how many AH's are consumed when running the A/C for the day????????????????????????

Also what fridge model do you have? Thanks Steven that is one of the fridges I have been considering.

All I am trying to do is work out a real cost for me to set up a system to do everything my current system does plus run a fridge and A/C.

I don't run my fridge off of 12v and I consistently use between 100 and 110 AH per day. My readings are taken from a separate battery monitor and are correct. So if we add another 90AH for a fridge plus another ??????????? for the A/C. I would need a system of at least ??????

Cost of batteries ???? Then another $3000 to $7000 for a new combo charger / inverter. In my situation I would need this because the only advantage of me changing to lithium is to operate the A/C. The current combo unit is a Heart interface 1000watt msw inverter / 50amp charger. Which will not handle the A/C.

So am I close? Not counting the monitoring equipment or wiring etc have I missed anything?

[BruceS]

OK, take a breath! Easy.........

Craig I recently bought a meter that plugs into a power point that you then can plug in the A/C to record various info that would tell you useage in a given period.
From memory it was only $32.90 or so.
http://cgi.ebay.com.au/ws/eBayISAPI.dll ... OC:AU:3160
Is your A/C wired direct or does it plug into a power point?
Surely you could get a rough estimate off the label on it or from the pamphlet that came with it.
Without this meter it would be guessing on cycle length and frequency.
Anyone done these tests?

[TigerMK1]

Why is it every time I ask a question about Lithium batteries I get a defensive answer and no one will answer the question I asked. I already now that AGM's will not support the types of loads that the lithiums will. I stated that in an earlier post so you don't need to prove that to me.

I would be interested in knowing how many AH's are consumed when running the A/C for the day????????????????????????

Also what fridge model do you have? Thanks Steven that is one of the fridges I have been considering.

All I am trying to do is work out a real cost for me to set up a system to do everything my current system does plus run a fridge and A/C.

I don't run my fridge off of 12v and I consistently use between 100 and 110 AH per day. My readings are taken from a separate battery monitor and are correct. So if we add another 90AH for a fridge plus another ??????????? for the A/C. I would need a system of at least ??????

Cost of batteries ???? Then another $3000 to $7000 for a new combo charger / inverter. In my situation I would need this because the only advantage of me changing to lithium is to operate the A/C. The current combo unit is a Heart interface 1000watt msw inverter / 50amp charger. Which will not handle the A/C.

So am I close? Not counting the monitoring equipment or wiring etc have I missed anything?

What you have not stated is your required running time for your aircon. On another post a lithuim user states his battery capacity and his aircon usage. If you want 24/7 aircon, get a big genie and camp well away from others, cheapest way. If you want to be slightly green, use limited aircon to take the peak off or change to efficient fans. Just had a really bad thought, open the fridge and cool the motor home. The constant running of the fridge can be calculated and you therfore have your energy requirements.