Pre-charging with resistor.

B.T.

New Member
Seeing some posts about using a resistor to pre-charge (the capacitors?) the inverter. I have an MPP 2424LV-MSD that I hope to have batteries for in the next few days. I have seen lots of differing opinions on the subject. I read in one place that if the inverter is small, under 3000 watts was their threshold, there is no need. The manual for the MPP does not mention it and as well I have seen Will just touch the cable to the battery and let 'er spark. I have a circuit breaker so I will not need to go through that, but I want to be as kind to my unit as possible. I have also noticed that the resistor values suggested are all over the place as far as ohms and watts. I have an old piece of electrical equipment that has a 5w 5ohm resistor. Can I use that? Is it just a matter of how long it will take to pre-charge? I am using (2) 12v 100Ah batteries in series with 2 gauge cable. (Originally purchased 2/0 cable but soon learned that the 2424LV will only accept 2 gauge max.) I should only be using about 16"-20" for the individual pos and neg terminals to the 2424LV. Any information would be appreciated.
 

JoeHam

Photon Sorcerer
If you leave your battery connected this will be a one time event and I wouldn’t worry too much.

There is no harm in trying the resistor you have on hand. Others have successfully used incandescent light bulbs.

Here is what I use since I disconnect often:

https://www.amazon.com/gp/product/B07MW4VHDB/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

However, I have “sparked” my 5 year old inverter more times than I will admit before I learned about precharging 😎
 

newbostonconst

Solar Addict
There are many different type of inverters and many different types of Batteries. Some don't need anything you just need to try it.

I have BattleBorn batteries that when I use them I have to pre charge but now that I have DIY Cells I do not.

Try without the resistor you have and if it doesn't work try with the resistor and if that doesn't work try with an even lower resistor.
I only had to hold on for a couple seconds before throwing the breaker.
 

JoeHam

Photon Sorcerer
I think all LFP batteries will spark when you connect to an inverter with discharged caps.

That is true of all my DIY and commercial batteries (6 different ones).
 

B.T.

New Member
Thanks for your replies. As I mentioned I have a breaker between the batteries and inverter so I will have that in the off position when I hook up the batteries and sparking shouldn't be an issue. I do plan however to remove the batteries from the camper when it will be stored for more than a few days, especially in the heat of the summer. I will likely just purchase a resistor next time I order something. I want to make my unit last as long as possible at least when it comes to proper maintenance and use.
 

B.T.

New Member
If you leave your battery connected this will be a one time event and I wouldn’t worry too much.

There is no harm in trying the resistor you have on hand. Others have successfully used incandescent light bulbs.

Here is what I use since I disconnect often:

https://www.amazon.com/gp/product/B07MW4VHDB/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

However, I have “sparked” my 5 year old inverter more times than I will admit before I learned about precharging 😎
Thanks, I had those exact ones on my wishlist so I appreciate the confirmation.
 

jtaon001

New Member
If you leave your battery connected this will be a one time event and I wouldn’t worry too much.

There is no harm in trying the resistor you have on hand. Others have successfully used incandescent light bulbs.

Here is what I use since I disconnect often:

https://www.amazon.com/gp/product/B07MW4VHDB/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

However, I have “sparked” my 5 year old inverter more times than I will admit before I learned about precharging 😎
Hey guys! So I'm going to wire up my inverter and 50 Ah battery with a 3 pin rocker switch today. So I can have one side to pre-charge with the 50 watt 25 ohm resistor you linked. Just to confirm, is this diagram correct?

What would be the easiest way to charge the battery? If the toggle switch is in the off position, can I just connect a trickle charger directly to the battery posts?

Thanks for your guys input!
 

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JoeHam

Photon Sorcerer
Make sure the switch is beefy enough to handle full surge current to your inverter.

Trickle charger to battery posts should be fine. Just make sure the inverter wires have a good connection not disturbed by the charger connection.
 

jtaon001

New Member
Make sure the switch is beefy enough to handle full surge current to your inverter.

Trickle charger to battery posts should be fine. Just make sure the inverter wires have a good connection not disturbed by the charger connection.
The switch is rated for 120V and 10 amps. The inverter will be under a continuous 1200 watt load, so about 10 amps. Should I switch it out with a 20 amp switch or even better at 50 amp switch? hahah!
 
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JoeHam

Photon Sorcerer
The switch is rated for 120V and 10 amps. The inverter will be under a continuous 1200 watt load, so about 10 amps. Should I switch it out with a 20 amp switch? haha

Better check your math. 😎

Clue, the switch voltage is 12v if I am reading your schematic correctly.
 

jtaon001

New Member
Better check your math. 😎

Clue, the switch voltage is 12v if I am reading your schematic correctly.
uh oh. So it's 120 watts? I'm starting to get confused HAHA. Does it matter that I will be plugging in a pump that is 115V x 10 amps to the inverter?

In my head, I'm figuring that the battery is 12 volts and 50Ah DC. I should probably put a switch rated for at least 50 amps then.

HELP! hahah
 

GSXR1000

Solar Addict
The switch is rated for 120V and 10 amps. The inverter will be under a continuous 1200 watt load, so about 10 amps. Should I switch it out with a 20 amp switch or even better at 50 amp switch? hahah!
1200w at 12v will be over 100 amps
 

JoeHam

Photon Sorcerer
Your inverter will put out 1800w continuous, 3000w surge!

I will use a 2400w load so I don’t need to bust out a calculator.

2400w divided by 12v is 200 amps.

That’s why many of us step up to 24 or 48v systems.

Glad I warned you about that 10 amp switch before you let the magic smoke out of it. 😎
 

GSXR1000

Solar Addict
all. batteries will spark...
the problem isn't the battery it is the BMS. you will have 100-10000 amps for a few microseconds.. which can trip a bms... maybe damage the mosfets
 

Bud Martin

Photon Sorcerer
You are using 25 Ohms to pre charge the capacitor banks in the inverter.
The 25 Ohms resistor will limit the dead short (discharged capacitor is like a dead short when Voltage is applied to it) current to 12V/25 Ohms = 0.48A. Your switch is fine.
Remember that resistor is connected in series with the load so it will limit the current flow.
 

GSXR1000

Solar Addict
watts=volts x Amos
so 12v is easy.... factor of 10
120v. 12v
1A. 10a
2A. 20a
10a. 100a
15a. 150a
30a. 300a
50a. 500a
 

GSXR1000

Solar Addict
You are using 25 Ohms to pre charge the capacitor banks in the inverter.
The 25 Ohms resistor will limit the dead short (discharged capacitor is like a dead short when Voltage is applied to it) current to 12V/25 Ohms = 0.48A. Your switch is fine.
Remember that resistor is connected in series with the load so it will limit the current flow.
look again... he is using the switch to hold the entire load also
 

JoeHam

Photon Sorcerer
Precharge is the least of your problems IMHO.

Step 1. Add a fuse or DC breaker very near the battery to your schematic.

Step 2. Make sure your wire size can handle the current:


Step 3. Get the appropriate size fuse or breaker. Remember, this device is to protect the wire from catching on fire.

Step 4. Come back with more questions.

Step 5. Forgive me for being a smart a$$. Mom says I am perfect proof no one likes a smart a$$ 😎
 
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Ampster

Renewable Energy Hobbyist
The precharge current is small depending how high a resistor one uses. A small switch can be used for the precharge. The main switch should be rated for the draw on the inverter. The complexity of the circuit would depend on how often you are connecting your battery and inverter. In a DIY EV the circuit turns on and starts a timer. The timer is adjusted for the time needed to precharge the caps, then the main contactor closes. For an inverter you are connecting once a month a resistor on alligator clips would work. When precharged just throw the main battery switch.
 
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