Lithium batteries and microwave use

[12thgenusa]

I am considering replacing four GC2 batteries with two 100 Ah LiFePo batteries with 100 Amp BMS each. My concern is whether they will be able to operate the microwave for light duty use (5-6 minutes) without voltage sag causing problems. The draw for the microwave is about 125 to 130 amps, depending on how much solar is coming in. The wiring is appropriately sized for the loads.

I currently have four GC2s for this reason, not because I really need that much storage capacity. The LAs have served well; they are going into their 12th season and still have very good hydrometer readings. However, I know their time left is limited.

Does anyone have experience operating the microwave with 200 Ah of lithium?

[otrfun]

12thgenusa wrote:
. . . My specific question is, are the characteristics of lithium batteries sufficiently different from FLAs that 200 Amp hours of lithium batteries with at least a 200 amp BMS will operate the microwave without voltage sag causing inverter under-volt alarm or shutdown?

Should be, is not a helpful answer.

Here's a lifepo4 C rate discharge curve at 25c (77f) that may help you better understand the voltage drop that occurs while powering a microwave with a lifepo4 battery. Multiply the voltage in the graph by 4 for a 12v lifepo4 battery. Smaller .7cf microwaves will draw approx. .5c using a 200ah lifepo4 battery. The average microwave will draw .6c - .75c.

As a lot of folks have already confirmed, a 200ah lifepo4 battery with a 200a continuous rating will power a microwave. The only thing that can derail your plan is voltage drop due to the use of too small of cabling. As long as the voltage drop between the battery terminals and the input to the inverter remains below 2% (at 150a) you should be able to access almost the full ah capacity of the battery. Inverters typically start experiencing low voltage alarms (and cut-off) when the input voltage drops below 11v. Assuming less than 2% voltage drop, you should be able to power a microwave with a 150a draw down to 5-10% SOC with a 200ah lifepo4 battery before the inverter low-voltage alarm activates. This equates to a bit more than an hour of runtime.

We use a 200ah lifepo4 battery to power our 11k BTU a/c unit (and small microwave). They both draw roughly the same current (105-110a via inverter). While powering the a/c, we've inadvertently discharged the lifepo4 down to low-voltage cut-off (2.7v at cell level or 10.8v overall) many, many times. After two years of doing so, we've never experienced an inverter low-voltage alarm or shutdown.

[StirCrazy]

12thgenusa wrote:
I’ll make my question a bit clearer.

When I designed and installed my system 12 years ago, lithium was still a dream for RVs. The battery discussions revolved around whether it was best to use a 12-volt battery or multiples of them, 6-volt batteries in series, or break the bank and go with AGM.

At the time, most agreed that a pair of GC2s in series were marginal to operate a microwave. That convinced me to go with four GC2s (440 Ah).

My specific question is, are the characteristics of lithium batteries sufficiently different from FLAs that 200 Amp hours of lithium batteries with at least a 200 amp BMS will operate the microwave without voltage sag causing inverter under-volt alarm or shutdown?

Should be, is not a helpful answer.

if you had a 200A BMS in one battery it would operate a microwave nice. two no problem.. the advantage of LFP is there is alomost no voltage sag undel load (there is but it is small) that is why you needed 4 GC instead of 2 as with only two the voltage sag when they get to about 80% of capacity will start your alarm going on the inverter. the LFP will stay above the alarm voltage untill they are down around 5ish % capacity.

[12thgenusa]

otrfun wrote:

12thgenusa wrote:
. . . Does anyone have experience operating the microwave with 200 Ah of lithium?

Important to note, Ah (amp-hour) and A (amp) are two different ratings.

A 200 Ah lifepo4 battery with a 100 A continuous current rating will not run the vast majority of microwaves. A 200 Ah lifepo4 battery with a 200 A continuous current rating will run most any microwave.

A similar caution applies if you have plans to parallel two 100 Ah lifepo4 batteries to obtain 200 Ah. Make sure *both/each* of these two 100 Ah batteries have a 100 A continuous current rating (*not* 50 A). Two 100 Ah batteries, each with a 100 A continuous current rating, paralleled together, will provide 200 A of continuous current.

That’s exactly what was stated in the OP.

[otrfun]

12thgenusa wrote:
. . . Does anyone have experience operating the microwave with 200 Ah of lithium?

Important to note, Ah (amp-hour) and A (amp) are two different ratings.

A 200 Ah lifepo4 battery with a 100 A continuous current rating will not run the vast majority of microwaves. A 200 Ah lifepo4 battery with a 200 A continuous current rating will run most any microwave.

A similar caution applies if you have plans to parallel two 100 Ah lifepo4 batteries to obtain 200 Ah. Make sure *both/each* of these two 100 Ah batteries have a 100 A continuous current rating (*not* 50 A). Two 100 Ah batteries, each with a 100 A continuous current rating, paralleled together, will provide 200 A of continuous current.

[deltabravo]

time2roll wrote:
My 4x GC2 needed 165 amps to run the MW. My lithium needs about 145 due

Actually it's the inverter that "needed" that amperage to run the microwave.
If the batteries "needed" amperage, they'd be charging.

In your scenario (batteries providing power to the inverter, to run the microwave) the batteries are discharging.

[3_tons]

You didn’t mention the size of your MW, but if it ran good using GC’s, you’ll have no problem with two LFP’s… Microwaves are a reactive (capacitive) load, and LFP’s are well suited for such loads because of very little voltage sag…Also, it’s not uncommon for a 100a/h LFP to be capable of discharging at up to 200amps for short periods of time (with time increasing at lower discharge amps…), meaning the amp hour rating (capacity) is not always indicative of its typically greater (amperage) discharge ability…

3 tons

[time2roll]

My 4x GC2 needed to provide 165 amps to run the MW. My lithium needs to provide about 145 due to the higher resting and loaded voltage. That pair of lithium will serve you well.

[wa8yxm]

Some numbers to consider.. MY JEEP
I have about 10 feet of #10 (10ga) ZIP cord between the battery (73AH Lead Acid) and the Rig Runner 5-40 (This is a power distribution box. 5 pair or anderson Power poles and fuses. the biggest one is fused at 40 amp. all pairs are rated 40 but may be fused smaller.

From there I use the stock factory 12 volt cable terminated in 40a amp andersons to my Kenwood TS-2000.. at 100 watts transmit power (About 20 amp draw) The radio shuts down due to low voltage.

Option 2 is a 12 AH LiFePO4 that's right 12 AH... I have a very short (like 6" or less) of 12 Ga wire and a pair or 40 Amp Andersons (We hams tend to use a lot of Andersons. I've a 40-12 Rig Runner behind me as I type. about half populate)

Full 100 Watts. NO voltage sag Im drawing nearly twice as many amps as the amp hour rating.

Your microwave may draw around 100 amps (100-120) or 1/2 the battery bank amp hour rating... I suspect it will work.. likel for around 1.5 Hours on a full charge.

[12thgenusa]

deltabravo wrote:


YEP.

Thanks

[deltabravo]

12thgenusa wrote:
At the time, most agreed that a pair of GC2s in series were marginal to operate a microwave.

My First RV had a pair of Interstate GC2 golf cart batteries and a 2000 watt inverter. I owned that rig for 7 camping seasons. Microwave always ran fine on that system.

[deltabravo]

12thgenusa wrote:
Does anyone have experience operating the microwave with 200 Ah of lithium?

YEP.

I had two Battleborns in my Nash 17k for about a year and a half. The Microwave worked fine on them along with the Victron Multiplus 2000 inverter. I think I installed them in early 2019.
In 2020 I sold that rig, and then bought a different truck camper and moved the whole works into my AF992 truck camper in late 2020.

Microwave still runs perfect with the same inverter and batteries.

I've had two other RVs with 200 amp hours of batteries and a 2000 watt inverter to run the microwave. They've all performed flawlessly.

[12thgenusa]

I’ll make my question a bit clearer.

When I designed and installed my system 12 years ago, lithium was still a dream for RVs. The battery discussions revolved around whether it was best to use a 12-volt battery or multiples of them, 6-volt batteries in series, or break the bank and go with AGM.

At the time, most agreed that a pair of GC2s in series were marginal to operate a microwave. That convinced me to go with four GC2s (440 Ah).

My specific question is, are the characteristics of lithium batteries sufficiently different from FLAs that 200 Amp hours of lithium batteries with at least a 200 amp BMS will operate the microwave without voltage sag causing inverter under-volt alarm or shutdown?

Should be, is not a helpful answer.

[greenno]

You should be able to use a microwave for short periods of time as well as a coffee maker if you choose.
It sounds like you already have some solar to replenish your bank so as long as it's somewhat sunny your good to go.

You didn't state it but Im sure you know you need an Inverter for the 120vac devices correct?

We have a bit more capacity at 560ahrs and I installed a 3kw Inverter for our AC loads when we need them.
Got 510w of solar on the roof as well so that pretty much tops us off when we do use the Inverter.
Good luck.

[LouLawrence]

Based upon the topic title I was going to say "Don't use lithium batteries in a microwave!" 🙂