DeanHensler

Hello everyone. I know this isn't RC related but you guys seem to have a good handle on Lipo batteries so I thought this would be a good place to get some help.

I've got a 24v cordless impact gun like this one: Trades Pro 24v Cordless Impact (sorry, this forum won't let me post links yet). You can google that to see the impact gun.

The impact is awesome and it has a ton of power for removing lug nuts from cars. The problem is the NiCd battery sucks. It doesn't last very long so I want to convert it to LiPo using some 18650 batteries I have from a laptop battery. I've done a lot of research on this and I'm down to a few final questions that I can't seem to find the answer to. Any help would be appreciated.

1. Using 6 of the 18650 Li-ion batteries in series I'll get between 22.2 and 25.2 volts. Will this be enough power for the 24v impact? I don't really understand how the voltage translates to power the impact is putting out. I know there are Amps and Watts to consider also, I'm just unclear on how all of that is calculated from the battery size.

2. I've seen the IMAX B6 charger and it looks great. However, it looks like it can only charge up to 6 cell Lipo batteries. If I need 7 cells to get the power I'm looking for, what charger is the best for that? Is there a specific reason the IMAX B6 only goes to 6 cells? Is 7 cells a bad thing?

Thanks so much for the help!

cutting42

Hi Dean

Firstly don't confuse a laptop Li-ion battery with a Lipo, they are not the same. The Lipo batteries used in cars have the ability to release very high power very quickly whereas a laptop battery needs to release much more slowly and have as large a capacity as possible.

Fortunately for you, a Li-ion is just what you need for your impact driver as well.

An 18650 cell has 3.6volts so you will need 6 or 7 cells as you mention, The driver will run just fine with 21.6v (6 cells), maybe a little slower but better than with Nicads.

For capacity, you need to know the mAh or milliamp/hour rating of the cell and also to make sure they are the same mAh rating. As you are connecting them in series to get to your correct voltage, you will have the same capacity for the whole pack so if each cell has a mAh of 3400mah then the whole pack has this rating. If you were to double the cells and run each cell in pairs to have the same voltage you would double the capacity to 6800mAh.

A picture of an example of series vs parallel (different voltages and mah but the principle is the same)



Charger I am not sure about as I have not used an RC charger for a Li-ion pack but the specs says it is fine to do this. Nothing wrong with 7 cell or any other number but you would need a different charger, this one does upto 8 cells (8S)
http://www.hobbyking.com/hobbyking/s...arehouse_.html

DeanHensler

Thanks for the response. CAn you or someone else explain the difference between a Li-ion and a LiPo battery pack? Is it a difference in the type of cells?

I'm also curious to understand how voltage translates to power that the impact gun will produce. I do clearly understand capacity and how that relates to how long the battery will last. That is the main reason for doing this conversion in the first place. I'm sure the NiCd batteries have low capacity, but there are enough of them in serial to have high voltage and give me the power I need.

I understand these 18650 batteries have a voltage range from 3.7v to 4.2v depending on the charge level. I would imagine having 7 cells at 3.7v to produce 25.9v would not burn up this impact. But having them fully charged at 4.2v each producing 29.4v might be putting too much power into this 24v drill. What do you think? How do you know when you're putting too much power into the device?

valk

iTrader: (13)

if it was me id prob just spend the money on a quality impact that is based on lipo technology. the batteries have LVC protection and will shut the drill down when voltages get too low. my milwaukee 18v drill, not even the impact, has more than enough snot to undo lugnuts and other bolts on cars. I picked them up for $189cdn on sale but you can get offbrands for less.
it just seems unsafe unless you can integrate a LVC switch.
lipo and li ion are similar technologies, but old school 18650 packs were super volotile and would explode in fireballs if treated badly. newer lithium polymer cells can handle higher current draws, ie you drill, without the tendancy to explode, but can still burn up if you draw lower than their minimum voltages.

stick to prismatics for an application like this. you would be better to buy a couple of 2-3 cell lipo packs for rc cars at 40-60c and just string them together. you would have some difficulty soldering to a bare prismatic cell.
but then you still need to figure out a low voltage protection to protect yourself from the cells.
lipo cells cannot be discharged below 3 volts or you run the risk of fireworks or at least diminished performance.

you could also try LiFe cells, but they don't have the current draw capacity of a lipo. safer, don't explode as violently and less prone to abuse. but less powerful.

Dave H

Highly recommend adding a balancer harness to the pack for charging safety. Lithium cells can get rather exciting if overcharged. This page provides a reference, choose Show Balancer wire hookup on the first couple sections.

Script Asylum Lipo Wiring

cutting42

The differences are in the construction and there are many websites with very detailed explanations and also some confusing data. This seems good:

http://batteryuniversity.com/learn/a..._ideal_battery

Regarding power, it is a very simple calculation of voltage multiplied by amps. P (in Watts)= I(current in amps) * V (voltage)

The amps that can be delivered by a battery is calculated by multiplying the c rating which describes its ability to deliver current such as 20c, by the mAh rating so a 3400 mAh (3.4amps) 20c battery will be able to deliver a maximum of 68amps which multiplied by the voltage say 24v will equal 1,632 watts of power which is a lot but only for about 3 mins. The above is based on Lipo's which I know best but Li-ion don't mention their c rating but it will be lower as they are not designed for fast release of power.

However what is most important is the power that the motor in your impact driver is asking for It might say in amps or in watts in the spec sheet somewhere. As long as the battery can deliver that amount of current/power) then it will be fine and performance will be correct as the voltage is the critical element to deliver the power int he driver, the current just supports it and the capacity says how long it can do it for.

Basically, you need a battery to keep the voltage at its target for as long as possible. All lithium based batteries are good at this but they don't like being fully discharged so you need to make sure you don't fully drain the battery and charge it up frequently. It should not be discharged below 3v per cell or you may destroy the cell and leave it unable to recover so for a 6 cell pack not below 18v. The capacity should be a high as possible so aim for a 4800 mAh call and that will last for ages. I have 5Ah batteries in my Milwaukee and it goes weeks between charges.

DeanHensler

I will certainly be using a balancer. Can I create a pack with 6 sets of 2 parallel 18650's in serial and use a single 6s load balancer? Will the charger know what the hell is going on?

Where is a good place to order battery connectors and balancer harness's for this?

GerryH

iTrader: (26)

Sony, probably one of the biggest name brand suppliers of 18650 cells, rates the discharge at only 10a. But I know my Ryobi 18v Li-Ion battery uses this type of battery without issue and I'm pretty sure so does everyone else.

I've rebuilt one 18v Ni-Cd battery with a 5s lipo battery. It was fairly easy because it already had the balance connector (which I just created a opening in the battery to pull it out when needed).

The only thing you need to be careful about is over discharge, since factory li-ion packs have low voltage cutoffs to prevent damage. You'll just have to know when to recharge.

Is this a tool you use frequently or just once in a while? I usually leave my converted battery at storage voltage since it can be weeks or months in between uses. I could definitely tell when it was getting low on the impact wrench.

I think you'll have plenty of voltage with only 6s.

DeanHensler

I use this once per week at the race track. What about the 6 sets of 2 parallel 18650's in serial and use a single 6s load balancer? Will the charger be okay with that?