diy solar

diy solar

Batteries vs Inverter size

OldJimbo

New Member
Joined
Sep 20, 2021
Messages
58
I have two 12v 100ah lifepo4 batteries in parallel (50a max thru the BMS) which I want to use for an inverter. Most of the stuff in my van is 12v, but I’m thinking of AC to run some tool chargers (makita, hitachi, etc) and a 24” tv with a DVD player. My thought is to get a 1500 to 2000w inverter so I can power an induction hot plate or small microwave. I will let the inverter control (or decide) what I will use with it since a cpap and a refrigerator are my only “must have’s”. So my question is: am I limited to 1000 watts output by only having 100a of battery power available? If so, can I just buy a 3rd matching battery to make a 1500w psw inverter practical? It would sure help to power the potential microwave.
 
100 dc amps * 12 volts low cutoff * .85 conversion factor = 1020 ac watts.

Yes you can add a 3rd battery in parallel to get ~150 amps continuous.
Since the current paths won't be exactly equal lets de-rate by .8 which gives you 120 amps continuous.
 
  • Like
Reactions: Zwy
Yes the pair of batteries will be a hard limit on the power. I would give at least 20% overhead for the largest draw item. I would have four of those batteries with 1500w and five with 2000w.
 
My 1000W microwave on a 2000W inverter uses 151 amperes from the battery while running. Probably has a small surge when starting.
 
Have you considered wiring them in series to reduce the current draw. You could use a 24 to 12 DC to DC converter for your 12 volt loads. I would certainly suggest 2s2p 24 volt over a 4p 12 volt system
 
Seriewiring isn't going to change the max available watts

2x 12V parallel / 50A each = 100A @ 12V = 1200w maximum
2x 12V in series / 50A max = 50A @ 24V = 1200w maximum

Both excluding the conversion losses, so you're sticking with the approx 1000w max

With a 3rd battery, you will get 12V / 150A, thus 1800w max. But it's unlikely all batteries will equally share the current, so its more likely to get 1300-1400 usable watts including conversion losses at max.

AC, induction and microwave... I won't recommend that based on the very limited batteries now available. With a battery only capable of providing 50A it's probably a cheap battery, which you're not even willing to push at full amps.
 
Why do you halve the maximum amps. If the BMS can handle a 100 amp continuous load, it can handle that load at 24 volts as well as 12. The battery’s capacity in watt hours isn’t effected you are correct, but by reducing the current involved you can increase the possible throughput by 2x
OP: I have two 12v 100ah lifepo4 batteries in parallel (50a max thru the BMS).
Isn't It because each OP's battery has BMS that can only handle 50A even though the battery is rated at 100Ah.

Also I am trying to understand "but by reducing the current involved you can increase the possible throughput by 2x"
Can you please explain that?
English is my second language, so I am trying to understand 'throughput by 2x'.
 
OP: I have two 12v 100ah lifepo4 batteries in parallel (50a max thru the BMS).
Isn't It because each OP's battery has BMS that can only handle 50A even though the battery is rated at 100Ah.

Also I am trying to understand "but by reducing the current involved you can increase the possible throughput by 2x"
Can you please explain that?
English is my second language, so I am trying to understand 'throughput by 2x'.
yes, I see I misread that now. I still stand by the series suggestion. 2P assumes a perfectly equal distribution of the draw, that will not happen. One battery will have a lower IR and be subject to a higher current. Higher voltages at lower amperage’s also have the benefit of a lower c rate for your charge / discharge cycles
 
In parallel, you will draw 50A from each battery (load will see 100A @12V), in series you still draw 50A from each batteries, load will see 50A but twice the Voltage(24V) so same C rating of charge/discharge and same power (1200W).
Also in series, will the battreies have to be matched also?
 
yes, I see I misread that now. I still stand by the series suggestion. 2P assumes a perfectly equal distribution of the draw, that will not happen. One battery will have a lower IR and be subject to a higher current. Higher voltages at lower amperage’s also have the benefit of a lower c rate for your charge / discharge cycles

The amp differences that my two parallel 4s 12v LiFePO4 batteries see when under load is inconsequential, as in not enough to justify worrying about or tracking. After a bit more than a year in service, they are well balanced. Paralleled batteries that don't balance could be due to poor quality cells/batteries, bad wiring or poor design. Any three of those issues could also occur in a series connection.
 
My 1000W microwave on a 2000W inverter uses 151 amperes from the battery while running
Most of the stuff in my van is 12v, but I’m thinking of AC to run some tool chargers (makita, hitachi, etc) and a 24” tv with a DVD player. My thought is to get a 1500 to 2000w inverter so I can power an induction hot plate or small microwave.

What is the input amps of the induction hot plate?

For what you describe I’d stay 12V and manage-out the induction plate if it needs above 1000-1200W. A resistance hot plate might even make sense in my small mind.

I just can’t see spending on induction, then spending on batteries, 24-12V step-down converter, and a bigger inverter just to support an induction plate.
 
I thought there were a few posts in the past about DC powered hot plates. Skipping the inverter is a little more efficient.
 
I thought there were a few posts in the past about DC powered hot plates.
May have been- might be good for OP to look them up.

I was just thinking of this thread and it seemed like a lot of higher cost stuff to do just to support a 120VAC induction plate when even a lowbuck walmartha camping burner will cook eggs, heat soup, and handle 2/0 heat shrink tubes…
I’m not cheap but I have cost-aversion.
 
The induction cooker can wait, both it and air conditioning are frills. So I’m going to buy one more battery for 12v 3s with 150 amps max output. That means I will probably buy a 1200 or 1500 watt inverter - but even if I stay with two batteries for awhile, I will still buy one of those inverter sizes. I can’t afford Victron here, or similarly priced inverters; so which is better- Novopal or Giandel? I’m thinking of spending about $200 so something else in that quality/price range would also work. I’m really interested in opinions or suggestions on the inverter, so will appreciate all the help I can get.
 
The induction cooker can wait, both it and air conditioning are frills. So I’m going to buy one more battery for 12v 3s with 150 amps max output. That means I will probably buy a 1200 or 1500 watt inverter - but even if I stay with two batteries for awhile, I will still buy one of those inverter sizes. I can’t afford Victron here, or similarly priced inverters; so which is better- Novopal or Giandel? I’m thinking of spending about $200 so something else in that quality/price range would also work. I’m really interested in opinions or suggestions on the inverter, so will appreciate all the help I can get.
Well my 1200W Pure Sine Giandel is going fine at ~4 years of use, 24x7 since April. So of course I’d recommend that.
 
Back
Top