What to know for this battery build ?


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Rodger Dodger

New member
Joined
Apr 6, 2022
Messages
5
I'm trying to build a batttery.

But I'm not sure how to decide how many will be needed in parallel and or in series ?

I'm trying to power and exsisting device that uses a 6 volt battery pack from 15 years ago but now I wish to power this device with LiFePO4
3.2 v batts.
The old bat pack gave about 2 hrs of burn time, I wish to get about 4 - 5 hours with this new pack.

What would I need to know and take into consideration to start this project ?

Of course I'm new to this game of electronic batt building but have some previous electrical experience.

All responses are appreciated.

RD
 

Korishan

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Joined
Jan 7, 2017
Messages
7,071
Welcome to the forum. Please look at the FAQ section to get a better understanding of how batteries work. Explanation of how series/parallel works and how to calculate your needs.

The numbers initially you'll need is the requirements of your device(s), voltage and amps. And then scale based on that.
 

Rodger Dodger

New member
Joined
Apr 6, 2022
Messages
5
I'm trying to build a batttery.

But I'm not sure how to decide how many will be needed in parallel and or in series ?

I'm trying to power and exsisting device that uses a 6 volt battery pack from 15 years ago but now I wish to power this device with LiFePO4
3.2 v batts.
The old bat pack gave about 2 hrs of burn time, I wish to get about 4 - 5 hours with this new pack.

What would I need to know and take into consideration to start this project ?

Of course I'm new to this game of electronic batt building but have some previous electrical experience.

All responses are appreciated.

RD
Where is the "FAQ" area located ?
I understand how "series" and "Parallel" work. No explanation needed there.
As I stated before the voltage of the previous bat pack only list 6v. But it does not mention of Amperage or watts required.
Is there not a way to calulate the unknown factors ?

Thank you, RD
 

Korishan

Moderator
Joined
Jan 7, 2017
Messages
7,071
Where is the "FAQ" area located ?
I understand how "series" and "Parallel" work. No explanation needed there.
As I stated before the voltage of the previous bat pack only list 6v. But it does not mention of Amperage or watts required.
Is there not a way to calulate the unknown factors ?

Thank you, RD
1649369715883.png


There isn't just series/parallel, but further explanations as well. Such as how to calculate the values you need to know for your build. How many cells in parallel, series, bms or not, comparisons with leadacid, etc.
 

Abson

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Joined
Mar 7, 2022
Messages
5
Before you try to build a battery pack, you should first calculate the daily power usage of the device (Wh), which is the premise of everything.
 

J_Mack58

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Joined
Apr 7, 2021
Messages
158
There is a scientific approach and experimental approach....sound like Roger just want to get going hell with it all.. LOL Here you go Roger, Get yourself four 26650 cells or some 18650's. Put them in a 2S2P configuration and charge them to 80%. Power your device and time how long it stays on. While it is on if you have a DC Clamp on meter measure the amp draw. Whatever that amp draw is times how many hours you want your device to be on is the "ampere-hour" battery pack you will build. A less experimental approach will be find the label on your device and it may tell you everything you need to know.
 

Rodger Dodger

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Apr 6, 2022
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This is a portable lighting device from years back. Neither the bat pack no the light it self has any information other then the bat saying "6v". That's it. So that idea is out.
I've got both 3.7 18650 Lithium ion bats and 3.2 26650 Lifepo bats sitting on the self.

JMack, your saying to power the device with the new configured 2s2p pack charged to 80% and time how long it stays on while I've got an amp meter clamped on it ? I that correct ?
Once I do that, then lets say I decide I want a (2 amp x 6 hour bat) or 12 ampHr bat as you put it, how am I going to know how to build that bat ? ,
And am I going to have to make sure to charge at a 80% limit every time I charge then connect it ?

One of my worry is (since I'm not a electronics guy) shouldn't I know the High and Low voltage and amperage limits of the bulb of this device before I decide to connect a new built bat pack ? ? .
Replacement parts are not available any longer.

Considering that 3.7 2s2p = 7.4v
3.2 2s2p = 6.4v
Couldn't the 3.7's be charged to a higher voltage or is that their limit ?
Or same with 3.2 Lifepo's

RD
 

floydR

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Joined
Aug 23, 2017
Messages
1,527
The 3.7V charges up to 4.2V (8.4V), the LiFePO4 3.2V charges up to 3.65V (7.30V)
later floyd
 

Redpacket

Active member
Joined
Feb 28, 2018
Messages
1,457
As others have said, each battery type has a nominal voltage & a minimum & maximum.
LiFePo4 is nom 3.2, min ~2.9, max 3.65V (better max 3.45 for longer life)
Li-Ion is nom 3.7V min ~3V, max 4.2V (better max =~4.1V for longer life).

So for a simple lighting application 2x LiFePo4 in series like J_Mack58 suggests will likely be fine.
 

J_Mack58

Member
Joined
Apr 7, 2021
Messages
158
This is a portable lighting device from years back. Neither the bat pack no the light it self has any information other then the bat saying "6v". That's it. So that idea is out.
I've got both 3.7 18650 Lithium ion bats and 3.2 26650 Lifepo bats sitting on the self.

JMack, your saying to power the device with the new configured 2s2p pack charged to 80% and time how long it stays on while I've got an amp meter clamped on it ? I that correct ?
Once I do that, then lets say I decide I want a (2 amp x 6 hour bat) or 12 ampHr bat as you put it, how am I going to know how to build that bat ? ,
And am I going to have to make sure to charge at a 80% limit every time I charge then connect it ?

One of my worry is (since I'm not a electronics guy) shouldn't I know the High and Low voltage and amperage limits of the bulb of this device before I decide to connect a new built bat pack ? ? .
Replacement parts are not available any longer.

Considering that 3.7 2s2p = 7.4v
3.2 2s2p = 6.4v
Couldn't the 3.7's be charged to a higher voltage or is that their limit ?
Or same with 3.2 Lifepo's

RD
Roger, all the cells you have has a milliamp-hour (mAh) capacity rating. From my limited cell experience the lowest I have dealt with is 1500 mAh cell. So all the same that cell will output 1.5 amps for 1 hour at the nominal voltage of 3.6 volts. Find what your cells mAh capacity is. Here on this site is the best "Battery Tools" in the world for looking up the mAh for the cells you have. once you know the capacity (mAh) then you parallel them to the desired run time. Example you want 12 amp-hour pack and your cells are 2000 mAh cells ... 6 in parallel will do it in theory but old used cells may not give 100% capacity they are rated for so go 7 in parallel. I would build this pack for your light using the 3.2 Lifepo's 2 in series and I can't tell you the parallel without knowing the mAh ... My 26650's are 3.2 volt, 3200 mAh so I would build a 2S4P. 4 times 3.2 amp-hours is 12.8 Ah. You next step let us see your cells. I know what you thinking and the answer is HELL NO you can not charge the 3.7 cell to 6 volts. Highest I seen is 4.2 volts and we don't go to this 100% battery charge voltage because it will reduce the life of the cell. You need to use 2 in series to achieve the 6 volts.
 

italianuser

Member
Joined
Feb 25, 2020
Messages
546
JMack, your saying to power the device with the new configured 2s2p pack charged to 80% and time how long it stays on while I've got an amp meter clamped on it ? I that correct ?
Yes, correct
Once I do that, then lets say I decide I want a (2 amp x 6 hour bat) or 12 ampHr bat as you put it, how am I going to know how to build that bat ? ,
If you measured that two 2000mAh 18650cells in series charged at 80% last 6 hours, just make a two cell battery (positive with negative). If, instead, you measure that this battery only lasts 3 hours and you want to double the capacity then make two batteries the same as before (two 2S batteries) and the connect these in parallel (positive with positive and negative with negative). Make sure you use the same cells (or equivalent, that is same chemistry and and same/similar capacity = don't mix a 3000mAh and a 1500mAh, don't mix a ICR cell with an INR cell) and that you have them all at the same voltage (+/- 0.1V should be ok).
Note that the bigger the battery the bigger should be the cables. I use a 13AWG when I do my tests with a 55W 12V light bulb.

And am I going to have to make sure to charge at a 80% limit every time I charge then connect it ?
You should have a lithium battery charger where you can set the upper charge voltage.
Or, seen as I'm lazy, I'd just buy a small DC-DC step down board (I buy them on AliExpress) with the right wattage. This way you can charge the two 18650 in series to full voltage (8.4V) and regulate the DC step down to output 6V or maybe 6.2V. Always buy these boards with a higher nominal watt value of what you really need so they work at around 50% max load.

8.4V Battery <---> DC-DC Step Down (regulated to 6V output) <---> Light

Yes, there's a small waste of current using a converter.

One of my worry is (since I'm not a electronics guy) shouldn't I know the High and Low voltage and amperage limits of the bulb of this device before I decide to connect a new built bat pack ? ? .
The higher voltage. Usually a light will just dim out when voltage is too low.
 

paunboli2

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Joined
May 13, 2022
Messages
1
I'm also want to make a battery pack to power my laptop outside , hot much power does I need .
 

italianuser

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Joined
Feb 25, 2020
Messages
546
I'm also want to make a battery pack to power my laptop outside , hot much power does I need .
A notebook could use something between 20W and 40W. A standard I5 or I7 stay on the low side during normal operations and on the high side with intensive usage of CPU/GPU. Read the label on the transformer, if could be something like "OUTPUT 19V 65W", that's your starting point. Note that some notebooks have a power connector which carries an extra wire, a part from + and -, and won't work with a standard transformer, connector looks the same as the others, though.
 
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gpn

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Joined
Jan 21, 2018
Messages
254
A notebook could use something between 20W and 40W. A standard I5 or I7 stay on the low side during normal operations and on the high side with intensive usage of CPU/GPU. Read the label on the transformer, if could be something like "OUTPUT 19V 65W", that's your starting point. Note that some notebooks have a power connector which carries an extra wire, a part from + and -, and won't work with a standard transformer, connector looks the same as the others, though.
My macbook pro can easily use all 87w the adapter provides at full cpu and gpu usage. Or 10w with a minimal load and lower screen brightness and integrated gpu instead of discrete. So the use case of the laptop is just as important. But running off a portable 400wh pack -> inverter -> power adapter I can run for 3-6 hours without tapping into the internal battery for my "typical" usage. Yes I realize that I had a lot of conversion loss with the way that I did it. It was for science because I was curious! I used a victron phoenix 350va inverter and the oem power brick for the test.
 
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