Battery monitor/charger question

[Bobby F]

Originally Posted by JHinman

How does a person pre-charge capacitors in an inverter?

Corn18 found a nice way to do it in his thread a few days ago. It's a workaround, but (to me) it's a better workaround than undersizing cables to keep from oversurging your BMS.

https://www.irv2.com/forums/f54/why-...er-554864.html


(ETA: you only need to do this precharging when you've disconnected your battery from the inverter and go to reconnect it.)

[hclarkx]

Originally Posted by JHinman

How does a person pre-charge capacitors in an inverter? I do not see that in the user manual.

I can see how the readings would be different if I was wasting power by heating cables. That is something to look into. 10 amps at 14 volts is 140 watts, which seems like a lot. I don’t feel the cables getting warm, but I have not actually measured the temperature.

I am planning on getting another battery and re-wiring with 4/0 cable. What I have seems to work, but if I can make it “better” I will do so just because I can.

I really appreciate all of these comments. It is really educational!

Perhaps Battleborne will get back to me tomorrow with their thoughts.

Not using 4/0 cable is a really poor way of limiting the capacitor inrush current! As you are aware, resistance is needed to limit the capacitor inrush current but higher resistance cables is not the answer.

Clearly 1/0 doesn't limit capacitor inrush current enough to keep two batteries from disconnecting via their short-circuit protection. Discharged capacitors are a short-circuit on the battery when first switched on, with current limited only by the cable resistance and any other resistance between the inverter and the battery (and a smidgeon of battery internal resistance). This short-circuit lasts only a millisecond or so but because the LiFePO4 batteries can deliver thousands of amps into a short-circuit, the BMS, for safety, is set to open very very quickly on currents of a few hundred amps or more.

To your question, pre-charging the inverter capacitors is done by bridging the switch or circuit breaker used to "energize" the inverter with a resistor of a few ohms. For instance, a 5 ohm resistor will limit current to about 13/5 amps (about 2.5 amps). That will "charge" the capacitors in a few seconds. A one ohm resistor will do this more quickly with about 13 amps max. The current jumps up to 2.5 or one or what have you and decays over a second to zero as the capacitor voltage comes up to battery voltage (13.3V).

If you only occasionally need to charge the inverter capacitors, you can do this by just holding the resistor in your hand and touching the leads to either side of the circuit breaker that will be closed to energize the inverter for a couple of seconds before that breaker is closed. If this needs to be done often (you will be fully unloading the batteries for storage now and then), you can buy any kind of normally open push button and wire it, in series with the resistor, across the breaker to make this a push button operation.

A third battery will share some of the high "inrush" to the capacitors and might let the three BMSs handle the high current, but maybe not. And more likely not when you upgrade the wire to the inverter to allow full inverter output without overheating. In any event, it's best to bring the caps up slowly with a resistor. This is easier on the capacitors. Most users use a 10 Watt resistor, not because the power is needed, it's just larger and has longer leads so is easier to handle.

And, if you intend to use that "3000" inverter, you will want a third battery just to 1) have the capacity to run the microwave as long as needed and 2) hold voltage up under that high load when the batteries are low and 3) keep the BMS from disconnecting when you draw over 200 amps (the 3000 inverter will take about 272 amps when fully loaded. Check me on this, but the BB 100's might be limited to 100 amps each.

As well, if/when you want to put more than about 2000W of load on the inverter, you will want more than 1/0 cables feeding it (they are good for about 170 amps). Adding a second 1/0 is often the easiest way to handle more current to the inverter (two 1/0 is good for about 340 amps while your inverter will never exceed about 300 amps).

[Bobby F]

Originally Posted by hclarkx

Not using 4/0 cable is a really poor way of limiting the capacitor inrush current!

It shocks me a bit that I've heard multiple people say that Battleborn techs told them to undersize their cables to avoid the BMS issue.

[JHinman]

Interesting comments, hclarkx!

Now that I know how to pre-charge the capacitors I am sure I will never do it. My routine is to get up in the morning before my wife wakes, turn on the inverter with the app on my phone so I can make breakfast, and turn it off again with my phone. No need to get dressed, go outside, and open the compartment with the inverter. So far that has been working. Of course, my demands on my inverter are probably unusually low.

With my existing configuration, which I have used for half a dozen trips over the last year, I have to be careful what I operate at any given time. Microwave and heat are OK, but microwave and toaster are not OK.

With my new 5er I may gravitate to using the inverter more, so plan to expand the battery bank. Microwave and toaster might still stress the system.

Your comment about fully unloading the batteries for storage caught my attention. Battleborn recommends discharging to 80% SOC and disconnecting loads and sources for storage. I do not recall anyone recommending complete discharge. Am I mis-understanding something here!

Thank you for your detailed post. Information like this helps me better understand what I am dealing with.

[JHinman]

Response from Battleborn (Dragonfly Energy) is as follows:

I don't think the readings are so far off. The inverter is going to give you the reading directly from the inverter where as the monitor reads the amperage where it passes through at the shunt, the charge will have to travel through wiring and depending on how long the run is to the shunt there will be resistance regardless, this seems like a normal difference to me.

Sounds like resistance in the wire is the culprit, as suggested by several of you.

By the way, I have another battery and a couple hundred dollars worth of 4/0 cable and terminals on order. I’ll see what that changes!

[hclarkx]

Originally Posted by Bobby F

It shocks me a bit that I've heard multiple people say that Battleborn techs told them to undersize their cables to avoid the BMS issue.

Likewise. Mind boggling. Trade one problem for another.

[hclarkx]

Originally Posted by JHinman

Interesting comments, hclarkx!

Now that I know how to pre-charge the capacitors I am sure I will never do it. My routine is to get up in the morning before my wife wakes, turn on the inverter with the app on my phone so I can make breakfast, and turn it off again with my phone. No need to get dressed, go outside, and open the compartment with the inverter. So far that has been working. Of course, my demands on my inverter are probably unusually low.

With my existing configuration, which I have used for half a dozen trips over the last year, I have to be careful what I operate at any given time. Microwave and heat are OK, but microwave and toaster are not OK.

With my new 5er I may gravitate to using the inverter more, so plan to expand the battery bank. Microwave and toaster might still stress the system.

Your comment about fully unloading the batteries for storage caught my attention. Battleborn recommends discharging to 80% SOC and disconnecting loads and sources for storage. I do not recall anyone recommending complete discharge. Am I mis-understanding something here!

Thank you for your detailed post. Information like this helps me better understand what I am dealing with.

The capacitors only need pre-charging when you close the circuit breaker to make power available to the inverter. After that, you can turn the inverter on and off as often as you wish and the capacitors will stay charged. The capacitors are "upstream" of the circuitry that is controlled by the on/off switch so remain energized as long as the circuit breaker (or fuse) between the inverter and the batteries remains closed.

By charging the capacitors with a resistor, you are fine with two batteries or one battery in that regard. More battery is only needed if you want to draw more than about 2000W from the inverter (if my recollection that BB's 100 Ah batteries are limited to 100 amps each).

By unloading the batteries, I meant remove all loads from them. This means open circuit breakers so that all house parasitic loads and, ideally, the inverter, are removed from the battery while in storage. I.e., the battery current is zero, no charging and no load. As such nothing can run down the batteries while in storage.

As for the inverter, the caps will draw very very little current, but I would still separate the inverter from the batteries during storage largely because with the inverter live (up to the circuits controlled by the inverter switch), any issue, breakdown in insulation mostly, could otherwise cause a fire. Or a leaky capacitor could drain the battery over time. It's just safer to have nothing on the battery while in storage.

Most LiFePO4 battery makers recommend discharging to around 50%. I don't know what's up with BB's 80%. I had not heard that. But, 80% is a lot better than 90% and above where there is some continuous "chemical activity" that takes a small but measurable toll on the batteries while in storage. The paper where I read about storage didn't differentiate on cell type. BB uses cylindrical cells while most LiFePO4 batteries are now prizmatic cells. Maybe there's some difference there. In any event, it seems 80% is not much different from 50 or 60% but much difference from above 90%.

Glad I could help.

[hclarkx]

Originally Posted by JHinman

Response from Battleborn (Dragonfly Energy) is as follows:

I don't think the readings are so far off. The inverter is going to give you the reading directly from the inverter where as the monitor reads the amperage where it passes through at the shunt, the charge will have to travel through wiring and depending on how long the run is to the shunt there will be resistance regardless, this seems like a normal difference to me.

Sounds like resistance in the wire is the culprit, as suggested by several of you.

By the way, I have another battery and a couple hundred dollars worth of 4/0 cable and terminals on order. I’ll see what that changes!

That's disturbing. I don't know what BB is thinking. Resistance is irrelevant. The circuit (i.e., wire) from the Victron Multiplus to the battery carries the current the Victron Multiplus is measuring and that's the same current flowing through the battery and to ground through the BMV712 shunt (which I presume is on the ground side of the battery though this doesn't matter). If no other wires connect to the conductors/terminals along the path from the Victron Multiplus to the battery ground or they are not taking any current or injecting any current, then the current is the same all along this path. And it is not affected by resistance along that path. The current will cause a voltage drop in that resistance, and devices measuring voltage at different points along the path will read different voltage levels, but the current will be the same at all points along the path. You mentioned something drawing an amp and a half IIRC but the difference is much greater than that.

Given the supposed accuracy of the BMV-712, I would expect the Victron Multiplus to be in the same ball park accuracy-wise or at least not much worse.

Either there's something drawing off the 7 amps (IIRC) or some ammeter circuit is awry; probably the one in the Victron Multiplus (just my instinct given the proven history of the BMV-712). Something like the solar controller (if connected) would not explain the discrepancy. Or the load you mentioned.

This is disconcerting (to me at least).

[Tomahawk]

I'd believe the shunt.
The other unit shows:
AC input = 1394W
AC output = 34W
Output to Battery = 13.78*84.1=1159W
What happened to the other 200W?

[hclarkx]

Originally Posted by Tomahawk

I'd believe the shunt.
The other unit shows:
AC input = 1394W
AC output = 34W
Output to Battery = 13.78*84.1=1159W
What happened to the other 200W?

Unfortunately, devices like the Multiplus are very inefficient. Below 85% is not uncommon. Also, Wattage is calculated from voltage and current, so if either of those is not accurate, the Wattage won't be accurate. I agree, the BMV-712 is the one to believe.

Though I suppose a good clamp-on ammeter with built-in voltmeter would reveal which device is in left field very quickly (these start at about $30 though I'd have more confidence in one up around $70 or more and being a name brand).

[JHinman]

I have added a third battery, and re-wired the whole shebang with 4/0 cable. No change in the charge rate reported by the two different devices.

That was not what prompted adding a battery and re-wiring, so no big deal. I was just curious about it.

Everything works, which is a big relief. Next step is to see how much AC stuff I can run at the same time.