Battery behavior

Apologies in advance if this is well known info.

I successfully set up my system over the weekend (so excited!) and decided to try running a 1400 watt heat gun on full power. Was curious how long it would last. I ran it for about 12 minutes the first time and again for another 10 a little later

I noticed that while I was ruining the heat gun the battery charge level dropped to around 65%. But as soon as I turned the heat gun off it jumped back to 100%. I don’t get why this happened. Shouldn’t it just stay at 65%? And how could it possibly jump back to 100%?

my system:

• 160 solar panel renogy
• 40amp MPPT also Renogy
• 1500 watt pure sine inverter
• Valence 138 ah battery

So this indicates youre monitoring battery state of charge by voltage. You cant do this. You didn't drop to 65%... you had voltage sag. Which is normal. And another reason you dont do it this way. You monitor SOC by amps in/out through a shunt.

Ped said:

So this indicates youre monitoring battery state of charge by voltage. You cant do this. You didn't drop to 65%... you had voltage sag. Which is normal. And another reason you dont do it this way. You monitor SOC by amps in/out through a shunt.

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Thank you. Is SOC short for State Of Charge? I’ll look to see if I can monitor the amps instead

you say it is normal - is it preventable? Is it going to hurt the battery or the inverter?

Yes, SOC=State of charge. The amount of voltage sag is related to the load you are putting on the system. More load=more sag. At 1400 watts (not including inverter loses), you are drawing about 116 amps at the battery, that's going to create some sag. If you are going to run heavy loads on a regular basis, you need to add more batteries in parallel, or change to a 24 or 48 volt battery/inverter setup.

Mine does this too but I figured it had something to do with the load, which the experts have confirmed. I just did this test because Will always does it on his tests. I get like an hour and a half to over two hours out of one of those used BYDs. But when it hits the brakes it is not necessarily because the battery is drained.
What happens is the voltage from the battery drops SO low that the built-in BMS on the battery that an alarm goes of on the battery and that's that.
I kill the built-in breaker, wait a few and flip it back on. The SCC and Inverter (MPP 24v All-in-one) fire back up and there is the voltage seems fine with still plenty of battery to run the lights, charge laptops, run the fridge and toilet. But it can't keep pulling that heavy heater load.
So this morning I decided, SCREW THIS STUPID MEANINGLESS TEST! It's dumb, never ever ever will I run a heat gun or any heat element in my cabin design for and hour plus. So that test method tells me nothing about my setup when held against my real life and usage, which the system was particularly designed around. I decided what I really want to know is how my panels, my SCC and Inverter and how my battery will hold up running my fridge, toilet, lights, vacuum, devices, washer machine, water pump, the SCC itself, etc. Even if my wife needed to run the hair dryer for 5 minutes. How will it all hold up if I have a sunny day followed by a couple of overcast? Tests like that tell me something about how well I did on my design, estimates and purchases.
So that is what I will begin doing starting today.

Sorry for the long absence. I work at a small college and we have been quite busy moving courses online.

I don't know how to calculate what size shunt I would need. My battery is 138ah Valence - if this all pans out I'm likely to purchase another down the road. Would I need a 150 amp shunt and then later get a 300 amp shunt (presuming I add a second138ah)?

Many (most) of the shunts I've looked at come with a battery monitor. The Renogy MPPT that I bought has a blue tooth monitor too. Do I need to install the additional monitor that seems to come with the shunts?

Aili battery monitor works. Shunt size is based on your charge and discharge current. Bigger shunt for bigger loads just like a fuse.

Watts / voltage = amps

If youre planning on running 1000w inverter 1000 / 13v = 77A

The Valence U27 batteries can be charged at a max of 70 amps. A single battery is rated for 150 amps continuous, 300 amps surge. As others have mentioned, the shunt size depends on intended loads and charging current. I went with a 300 amp shunt since I knew I would want to be able to run my microwave off the inverter if the power was out. At full power, it draws 138 amps at the battery. Going with the larger shunt from the start gives me room to grow if needed and doesn't introduce a bottle neck in the system. I can run a single or parallel batteries and not have to worry about it.

Ah, thank you both very much!