Dual 100A LiFeP04 batteries with 2000 watt inverter.

[RichBenn]

I have a travel trailer. I was going to purchase 2 100aH batteries(each has a 100a BMS), but want to occasionally use a microwave with an inverter. The inverter would need to be 1500-2000 watts to power the Microwave(13 amps at 110V). This would mean drawing over 100 amps at 12 volts. Fine, I thought, I have 200 amps. But then I read somewhere about a “Ping Pong” effect. What that means is that one of the batteries BMS system would detect 100 amps and shut off. Then the second would take over, then detect over 100 amps, shut off, then the first would take over.

This would supposedly reduce the battery longevity, not to mention inverter or microwave stress.

Is this “ping pong effect” a real thing when using a large inverter? The solution would be one 200aH battery with a 200a BMS, but….

Edit - My initial alarm was a blog on an Australian site Enerdrive, which seems to be related to Dometic:

“The other question we always get asked is, what if I put 2 x 100Ah batteries together in parallel, can I use a 2000W inverter? Again we are talking about the Lithium batteries on the market that look like an “AGM” battery here.

As described above each battery has a maximum current output that can be achieved (100A per battery). The internal BMS setups have a safeguard built into their system. If a battery reaches/exceeds the maximum current output, the battery will switch off internally to protect the BMS and the cells from over discharge.

When this happens, the inverter and any loads running at the time are completely disconnected from the battery. Generally after 2-5 seconds, the battery BMS will switch the battery back on. If the large load is still present then the battery will just shut down again and the circle continues.

When you have 2 batteries in parallel and the above happens, the inverter/loads are transferred to the second battery, and in many cases will overload that battery and it too will switch off. By the time this happens, the first battery has switched back on and the loads return back to the first battery and again the circle continues.”

 

[BentleyJ]

If you purchase 2 quality batteries of the same type and cell chemistry they would have similar internal resistance as well as charge/discharge characteristics. If your Inverter is pulling 120A total then each battery would only be at about 60A. I'm running 2 batteries on my home back up system both are the same LiFePO4 chemistry but one is 150Ah while the other is 120Ah. I haven't had any kind of problem so far.

 

[BentleyJ]

Will posted this video today, I just found it. May answer some of your questions.

 

[RichBenn]

If you purchase 2 quality batteries of the same type and cell chemistry they would have similar internal resistance as well as charge/discharge characteristics. If your Inverter is pulling 120A total then each battery would only be at about 60A. I'm running 2 batteries on my home back up system both are the same LiFePO4 chemistry but one is 150Ah while the other is 120Ah. I haven't had any kind of problem so far.

Thanks. That’s what I thought. I just didn’t know how the BMS could effect the presented internal resistance. I’m going to post where I read this.

 

[jharrell]

So long as your wiring is done correctly and batteries are of similar capacity current will be shared and neither battery will "detect" more than half the current. A 100 amp rated BMS is going to be its continuous rating, it will typically have a much higher surge rating of 2-3x where it can handle higher than it continuous rating for a short period for say inductive load starting. Normally you would have to exceed the continuous rating for a significant time for them to shutdown if its slightly over they won't trip quickly until it's close to max surge.

Its really comes down to BMS quality and programming and what you surge currents are (starting an AC, older Microwaves). I have no issue running my microwave off my two SOK batteries with 100amp BMS's and my 3000w inverter, its not even close, I can even run the microwave off one of them, however SOK's BMS seems to be underrated they don't seem to trip until above 150 amps for 10 minutes and can handle 320 amp surge. I ran both A/C and microwave at same off both my SOKs pulling nearly 300 amps without issue (although I do have soft starter on A/C).

 

[RichBenn]

So long as your wiring is done correctly and batteries are of similar capacity current will be shared and neither battery will "detect" more than half the current. A 100 amp rated BMS is going to be its continuous rating, it will typically have a much higher surge rating of 2-3x where it can handle higher than it continuous rating for a short period for say inductive load starting. Normally you would have to exceed the continuous rating for a significant time for them to shutdown if its slightly over they won't trip quickly until it's close to max surge.

Its really comes down to BMS quality and programming and what you surge currents are (starting an AC, older Microwaves). I have no issue running my microwave off my two SOK batteries with 100amp BMS's and my 3000w inverter, its not even close, I can even run the microwave off one of them, however SOK's BMS seems to be underrated they don't seem to trip until above 150 amps for 10 minutes and can handle 320 amp surge. I ran both A/C and microwave at same off both my SOKs pulling nearly 300 amps without issue (although I do have soft starter on A/C).

That’s great With the SOKs. Sounds like they are a premium brand.

My “microwave” is actually a convection/speedcook microwave. Still1500 watts, and basically adds a resistive element for the heater, so no surge there. Maybe 13 amps, so no problem. Don’t really plan on any A/C use, unless I get a soft start and just use for a few minutes to take the edge off.

i posted this because of the info I referenced In the first post. Didn’t make sense to me unless the batteries were badly mismatched or one had a serious problem. So thanks for the real world answers.