Hi and a few Q's

[mattprinceuk]

Hi All,

Currently converting a Mercedes 814d bus to a motorhome and have a few questions.

I have calculated that I will be using 3974w per day (that's going overboard with everything and also using a powerful PC which will pull 750w, 4hrs a day) and want to know if I've worked my calculations out right?

2x200ah Lithium batteries will give me 4800w of usable power (400ah X 12v)
I want to get 4x250w solar panels (my vehicle roof is nearly 8m so plenty of room) so with the 70% efficiency from the panels, I will be able to recharge the 2 lithium batteries in just under 7hrs (1000w X 0.7 = 700w , 4800w/700 = 6.86hrs to charge)

Am I correct in this or am I missing something? Does using the PC off an inverter make any difference? Would I be better using a 24v system as the bus's own electrical system is a 24v system?

 

[mattprinceuk]

To clarify , the items that will be using electric are as follows.

12v LED Lighting spotlights in the roof
Water Pump for the shower
Charging the phones and tablet (using the original plugs connected to the inverter)
Maxx Air fans
PC (4hrs a day maximum as I am a video editor/colourist)

The items I have omitted are the fridge (will be powered by LPG as have a big long LPG tank underneath the bus), Cooker (powered by LPG) and heating (powered by diesel off the main tank)

Matt

 

[mattprinceuk]

Also would lifepo4 be a better route for me , cost wise if I am wanting 400ah?

 

[mattprinceuk]

Anyone?

 

[MisterSandals]

Looks like your math is sound but its hard to follow since you are confusing watts and watt hours.

Does using the PC off an inverter make any difference? Would I be better using a 24v system as the bus's own electrical system is a 24v system?

Anytime you convert from DC to AC with an inverter you lose ~15%. Your PC probably loses that same amount converting back to DC internally (or externally if your PC is a laptop). So that makes a difference.

Also would lifepo4 be a better route for me , cost wise if I am wanting 400ah?

LiFePO4 is usually a better route cost-wise for a long term investment. Note that lead acid will have about 50% usable power so should be double the "rated storage" of an LiFePO4 battery (which can be discharged nearly completely). And, LiFePO4 should have around 10x the number of charge/discharge cycles. The upfront cost is significantly more so you need to weigh the pros and cons for your situation.

 

[mattprinceuk]

Looks like your math is sound but its hard to follow since you are confusing watts and watt hours.


Anytime you convert from DC to AC with an inverter you lose ~15%. Your PC probably loses that same amount converting back to DC internally (or externally if your PC is a laptop). So that makes a difference.


LiFePO4 is usually a better route cost-wise for a long term investment. Note that lead acid will have about 50% usable power so should be double the "rated storage" of an LiFePO4 battery (which can be discharged nearly completely). And, LiFePO4 should have around 10x the number of charge/discharge cycles. The upfront cost is significantly more so you need to weigh the pros and cons for your situation.

Confusing watts and watt hours?

Been reading online the last few hours and think I am going to go the 24v route and get less battery ah as I’m right in thinking that a 12v system, the PC for example will use 250ah over 4hrs (750w/12v=62.5ah X 4hrs is 250ah) whereas a 24v system it will only use 125ah (750w/24v = 31.25ah X 4hrs is 125ah) ?

So I can the 24v solar panels and then get lifepo4 cells (8x3.2v 280ah) and make myself a 24v 280ah battery pack.

 

[MisterSandals]

12v system, the PC for example will use 250ah over 4hrs (750w/12v=62.5ah X 4hrs is 250ah) whereas a 24v system it will only use 125ah (750w/24v = 31.25ah X 4hrs is 125ah) ?

The amount of actual power used does not change based on the voltage. This is where watt hours becomes a useful measure of energy:
250ah x 12v = 3000wh
125ah x 24v = 3000wh

750w (at any voltage) x 4hrs = 3000wh

 

[mattprinceuk]

So if I was on a 24v system with 280ah I will have 6720wh and on a 12v system with 400ah I will have 4800wh so it’s still going to be better and cheaper for me to go 24v?

 

[MisterSandals]

So if I was on a 24v system with 280ah I will have 6720wh and on a 12v system with 400ah I will have 4800wh so it’s still going to be better and cheaper for me to go 24v?

Better and cheaper?
The advantage i see with 24v over 12v is when your 12v wiring needs to handle large loads (like 3000w) and requires very large wires and fuses to handle the amperage (3000w / 12v = 250amps). For 24v that is a more reasonable 125amps.

And, all of my RV systems and toys around the house are 12v so thats a no brainer for me.

Cheaper? Probably not, i think 24v electronic stuff is a little rarer than 12v but you don't need me to look up prices.

 

[mattprinceuk]

But going by my above post, going 24v, I will need half the amount of ah battery to have the same wh? If so then that’s quite a bit cheaper.

 

[MisterSandals]

But going by my above post, going 24v, I will need half the amount of ah battery to have the same wh? If so then that’s quite a bit cheaper.

I am not sure what cell pricing you have for your batteries but a 24v battery will require 8 cells and a 12v battery will require 4 cells.
24v 280ah battery: 8 x 280ah (cost per cell is?)
12v 400ah battery: 4 x 400ah (cost per cell is?)

 

[MisterSandals]

You may find a cheaper battery by a negligible amount in terms of $/wh but you should look at the big picture of your entire system and your use(s). Is your bus going to be AC appliances? If so, then going 24v will work better with a large inverter. If your bus will be 12v then that may be a handier solution.

The rule of thumb bandied about on this forum is to use 12v up until your loads (usually a big inverter) get up around 3000w.
If you go with a 24v system and your bus has 12v lighting and stuff, then you will need to down convert all of that from 24v to 12v (i have not done it but many others have).

My point is to look at what voltage you expect to use or which is better for your upcoming build. That is more important than fussing over $/wh which will likely be fairly insignificant compared to other things in your system.

 

[snoobler]

24V to 12V is pretty trivial. A simple 24-12V step down converter does the trick. A variety of options in output power:

https://www.amazon.com/Cllena-Converter-Regulator-Transformer-Waterproof/dp/B07QXV8V2L

 

[mikefitz]

With a 24 volt system, easier to integrate with the vehicle electrics, and MPPT solar controller is less expensive.
Use a custom 24v input PSU for the computer, example,

24V DC Input PC ATX Power Supplies, 24 volt Input, 600 watts Autonomous vehicle computer power supply from PowerStream, ATX DC-DC.

600W 24 volt DC Input ATX Computer Power Supply, for running tower PCs in solar systems, trucks, boats.

www.powerstream.com

You will need a 24 to 12 dc converter to power the camper stuff.

Or stay at 12v for the battery system and use a 12v input PSU,

High reliability 12V Input PC ATX computer DC-DC Power Supplies, 12VDC Input, autonomous vehicle computer power. car computer power supply, 12 volt, 320W, 450W, 500W, 650W, 1000W, 1500W, 2200W, ATX DC-DC

car PC power supply, ATX computer power supply with 12V DC Input,DC-DC-ATX, 12V to ATX Computer Power Supply, carputer, automobile computer ATX power.

www.powerstream.com

With two 200Ah 12v batteries, either a 12 or 24 volt system will have the same amount of power available.

It may be useful to have alternative charging if the solar is inadequate.

If you have room fit as many panels as practical,.

If you do decide to use an inverter, then a 1000w true sine wave model should be enough to power the computer and is practical on a 12v system.

8 off 280 Ah cells could be arranged as a 12 or 24 volt battery. Building your own battery is possible but you need the equipment, time and skills to do so.
Dont be mislead by the so called 12 and 24 volt panels, the actual nominal output is around 20 v and 40 v respectively. A quality MPPT solar controller be able to function with the panels arranged in a series/ parallel combination for either a 12 or 24 volt battery system.

Depending when durring the day your 4 hours of work time takes place, solar could provide most/all the computer power.

Mike

 

[mattprinceuk]

The bus is a 24v system but will be kept separate anyway from the solar and interior electrical load. 400ah on 12v is going to cost me £2000.
If going 24v will mean I need less capacity than that then the 280ah will cost me approx £1000. Thats a big saving. Either route will cost me half by going to 24v. 280ah 24v will cost me £1000. To get the same capacity on 12v will cost me double. Or am I looking at it wrong?

 

[mattprinceuk]

Based on my estimated usage (gone overboard) I would use 3974w a day. Going the 24v route and getting 8x 3.2v 280ah batteries to make a 24v 280ah battery, that should be more than enough shouldn’t it? 280ahX24v is enough to power 6720w. Is that right? If so that’s the route I’m going to take as will be cheaper.

 

[snoobler]

Based on my estimated usage (gone overboard) I would use 3974w a day. Going the 24v route and getting 8x 3.2v 280ah batteries to make a 24v 280ah battery, that should be more than enough shouldn’t it? 280ahX24v is enough to power 6720w. Is that right? If so that’s the route I’m going to take as will be cheaper.

This is what @MisterSandals was talking about. Do you mean Watts, or do you mean Watt-hours?

3.2V * 8 cells * 280Ah = 7168Wh

MOST LFP cells are limited to 1C discharge and 0.5C charge.

This would mean 280A out and 140A in.

280A * 25.6V = 7168W peak output
140A * 25.6V = 3584W peak input

 

[mattprinceuk]

Its probably better if I break it down and explain. Ive based the figures on this..

<A> PC uses 750w and want to use it for 4 hrs a day (4hrs continuous)
<B>Led Lighting are 2.8w each and I will have 10 of them all on for 8hrs a day continuous.
<C> Water pump for the shower uses 30watts and will be on for 1hr solid a day (5 of us showering)
<D>Maxx air fans use 120w and will have 2 of them, so 240w. These will be on for 4hrs.
<E> 5 phones charged daily. All take 2hrs to charge and all use an 18w charger.


What does that work out as? I don't really understand the difference between watts and watt hours

Matt

 

[snoobler]

Watt is POWER - the RATE of energy consumption. 1W is a Joule(J)/second. A Joule is a defined quantity of energy.
Wh is ENERGY - Power * the time used. Wh is an easier way to think of it rather than breaking it down to joules

1 Wh = 1 W used for 1 hour. If you break that down, J/s * 3600s = 3600J - not important beyond understanding that Wh is just another expression of energy.

a) 750*4 =3000
b) 2.8*10 * 8 =224
c) 30*1 =30
d) 240 * 4 =960
e) 2*18*5 =180

Sum: 4394
To account for inefficiencies, divide by 0.85; 4394/.85 = 5169Wh (this really only applies to the AC powered items, but it's always better to have more than less).

You will use 5169Wh of ENERGY per day.

Adding up the individual POWER of the AC items:

750+18*5 = 840W (assuming everything else is running directly from battery)

Your inverter needs to supply 840W of power if you have everything running at once.

Summary:
You need to store/replace 5169Wh of energy per day to run the items as you describe.
If all items are on at the same time, you need an inverter capable of outputting 840W.

 

[mattprinceuk]

Watt is POWER - the RATE of energy consumption. 1W is a Joule(J)/second. A Joule is a defined quantity of energy.
Wh is ENERGY - Power * the time used. Wh is an easier way to think of it rather than breaking it down to joules

1 Wh = 1 W used for 1 hour. If you break that down, J/s * 3600s = 3600J - not important beyond understanding that Wh is just another expression of energy.

a) 750*4 =3000
b) 2.8*10 * 8 =224
c) 30*1 =30
d) 240 * 4 =960
e) 2*18*5 =180

Sum: 4394
To account for inefficiencies, divide by 0.85; 4394/.85 = 5169Wh

You will use 5169Wh of ENERGY per day.

Adding up the individual POWER of the AC items:

750+18*5 = 840W (assuming everything else is running directly from battery)

Your inverter needs to supply 840W of power if you have everything running at once.

Summary:
You need to store/replace 5169Wh of energy per day to run the items as you describe.
If all items are on at the same time, you need an inverter capable of outputting 840W.

Yeah I understand that and was pretty much what I was trying to say. Getting a pure sine inverter to handle 840w will be easy. So I need to store 5169wh of energy. I will have 1000w solar panels so taking off the 30% due to inefficiency , that will only take just over 7hrs to store that , on a good day? Also will a 24v 280ah lifepo4 battery be capable to store 5169wh? I work it out as 280x24=6720. Is that 6720wh? If so then that will work.