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Old 04-11-2009, 12:58 PM   #1
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Default need help converting from NIMH to lipo

All I have for batteries are nimh batteries and I want to get into lipo. so what are the details guys? I know nothing about it but I always hear about lipo-cutoffs, not to run the battery down too much, balancing, discharging, proper chargers... the list goes on. What does all of that mean and what else do I need to know... like the difference between 2c and 3c. I want to know it all so I don't blow anything up
Thanks to all who give me some info.
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Old 04-11-2009, 01:36 PM   #2
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Lipo has 3 voltages that are mentioned - max cell voltage (4.2v/cell), nominal voltage (3.7v/cell) and the min cell voltage (~3v/cell). The min and max voltages are important becasuse if you exceed max or discharge below min there's potential for damaging the cell and runining it making it unsafe to use. Lipo uses a diff charge protocol or routine. Its not a peak type routine like NiMH uses, it is a CC/CV routine - the cells are charged a constant current until the cell maximum voltage is reached and to avoid overcharging cells, the charging routine switches to a constant voltage stage where the max voltage is held on the cells and current tapers off as the cells top off in capacity. Nominal is what is advertised - when someone says 7.4v 2S lipo they are talking about the nominal voltage even though when fully charged, that 2S will indicate 8.4volts (4.2v/cell x 2 cells).

The C business is the batt capacity divided by 1000, so for a 5000mah lipo, its "C" is 5amps (5000/1000). This is used to specify charge rate as in a 1C or 2C charge (for same 5000mah lipo, a 1C charge is 5amps and a 2C would be 10amps). The other way the C rating is used is for discharge capability. A 5000mah lipo with a 20C rating can discharge 5 x 20 or 100amps contniuously. Lipos are usually given continuous and burst ratings. These numbers should be based on testing, but since there is no industry standards, some 25C rating is another manufacturer's 20C rating, these can be inflated sometimes. A little like advertised nitro horsepower numbers.

Lipo are made using 2 or more individual cells. the S number is the number of cells wired in series - as in 4S - thats 4 3.7v cells assembled in series for 4 x 3.7 = 14.8volts nominal. Fully charged it would be 16.8volts. The 2nd number you see sometimes is the # of cells in parallel as in 4S1P or 4S2P. Adding cells in parallel adds capacity, but does not increase voltage. If a 4S1P was 5000mah capacity, a 4S2P would have same voltage, but 10,000mah capacity. Adding cells in parallel is usually done to increase run time.

Since lipo has more than one cell, the balance of cell voltages is important. If the cells are out of balance and you charge it, there is potential to overcharge the cell with highest starting voltage. Chargers look at total voltage and not individual cell voltages (there are chargers that charge cell by cell, but they are not common). Same for discharging or running with a lipo. A low voltage cutoff (lvc) looks at a total voltage like 6.2v so if the cells start off out of balance, one cell can be overdischarged and the lvc would allow it since combined voltage could still be more than the lvc. Balancing cells with a balancer minimizes possiblities of either overcharging or overdischarging cells.

Some chargers do not have builtin balancers, some do. Those that don't can use a separate balancer. Separate balancers can perform charge thru or standalone balancing. Better ones will do both. Charge thru will give you a little more complete charge since standalone balancing simply checks cell voltage and discharges the rest of the cells down to that lowest voltage. A charger with balancing capabilities does the balancing while charging getting all the cells as close to cell max voltage as possible.

Lipo cells vary in quality and cost. The most expensive cells are produced with rigorous QC and carefully matched with others on assembly. Cheaper lipos don't take these added steps. Cheaper lipos get out of balance easier and need more in the way of balancing than the high quality lipo. Even the best lipos eventually do need to be balanced to maximize life and performance.
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Old 04-11-2009, 01:41 PM   #3
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Quote:
Originally Posted by Duster_360 View Post
Lipo has 3 voltages that are mentioned - max cell voltage (4.2v/cell), nominal voltage (3.7v/cell) and the min cell voltage (~3v/cell). The min and max voltages are important becasuse if you exceed max or discharge below min there's potential for damaging the cell and runining it making it unsafe to use. Lipo uses a diff charge protocol or routine. Its not a peak type routine like NiMH uses, it is a CC/CV routine - the cells are charged a constant current until the cell maximum voltage is reached and to avoid overcharging cells, the charging routine switches to a constant voltage stage where the max voltage is held on the cells and current tapers off as the cells top off in capacity. Nominal is what is advertised - when someone says 7.4v 2S lipo they are talking about the nominal voltage even though when fully charged, that 2S will indicate 8.4volts (4.2v/cell x 2 cells).

The C business is the batt capacity divided by 1000, so for a 5000mah lipo, its "C" is 5amps (5000/1000). This is used to specify charge rate as in a 1C or 2C charge (for same 5000mah lipo, a 1C charge is 5amps and a 2C would be 10amps). The other way the C rating is used is for discharge capability. A 5000mah lipo with a 20C rating can discharge 5 x 20 or 100amps contniuously. Lipos are usually given continuous and burst ratings. These numbers should be based on testing, but since there is no industry standards, some 25C rating is another manufacturer's 20C rating, these can be inflated sometimes. A little like advertised nitro horsepower numbers.

Lipo are made using 2 or more individual cells. the S number is the number of cells wired in series - as in 4S - thats 4 3.7v cells assembled in series for 4 x 3.7 = 14.8volts nominal. Fully charged it would be 16.8volts. The 2nd number you see sometimes is the # of cells in parallel as in 4S1P or 4S2P. Adding cells in parallel adds capacity, but does not increase voltage. If a 4S1P was 5000mah capacity, a 4S2P would have same voltage, but 10,000mah capacity. Adding cells in parallel is usually done to increase run time.

Since lipo has more than one cell, the balance of cell voltages is important. If the cells are out of balance and you charge it, there is potential to overcharge the cell with highest starting voltage. Chargers look at total voltage and not individual cell voltages (there are chargers that charge cell by cell, but they are not common). Same for discharging or running with a lipo. A low voltage cutoff (lvc) looks at a total voltage like 6.2v so if the cells start off out of balance, one cell can be overdischarged and the lvc would allow it since combined voltage could still be more than the lvc. Balancing cells with a balancer minimizes possiblities of either overcharging or overdischarging cells.

Some chargers do not have builtin balancers, some do. Those that don't can use a separate balancer. Separate balancers can perform charge thru or standalone balancing. Better ones will do both. Charge thru will give you a little more complete charge since standalone balancing simply checks cell voltage and discharges the rest of the cells down to that lowest voltage. A charger with balancing capabilities does the balancing while charging getting all the cells as close to cell max voltage as possible.

Lipo cells vary in quality and cost. The most expensive cells are produced with rigorous QC and carefully matched with others on assembly. Cheaper lipos don't take these added steps. Cheaper lipos get out of balance easier and need more in the way of balancing than the high quality lipo. Even the best lipos eventually do need to be balanced to maximize life and performance.

WOW well said!
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Old 04-11-2009, 02:22 PM   #4
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Its not as scary as it sounds... You buy quality packs.. a decent charger (with or without a balancer).. and it's very easy to deal with.

All the scary stuff happened in the early days of LiPo when it was new technology in RC. The chargers today with the proper profiles as mentioned earlier make it quite simple. At my local track, its been 80% lipo for the past year. No problems at all.

It would be a good idea to use an ESC with a LiPo cutoff or add one. The best thing about LiPo is you can charge them, run them and charge them again.. and run the again.. back to back. I usuall just run the same pack from practice and throughout the race night. Qualifiers and the main. I just put the pack on the charger after each race and its ready to go by the next heat.

You add a brushless ESC/Motor system and there is almost nothing to do between races! You get a lot more driving time at the track!

Jerome
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Old 04-12-2009, 01:32 AM   #5
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Quote:
Originally Posted by Duster_360 View Post
Lipo has 3 voltages that are mentioned - max cell voltage (4.2v/cell), nominal voltage (3.7v/cell) and the min cell voltage (~3v/cell). The min and max voltages are important becasuse if you exceed max or discharge below min there's potential for damaging the cell and runining it making it unsafe to use. Lipo uses a diff charge protocol or routine. Its not a peak type routine like NiMH uses, it is a CC/CV routine - the cells are charged a constant current until the cell maximum voltage is reached and to avoid overcharging cells, the charging routine switches to a constant voltage stage where the max voltage is held on the cells and current tapers off as the cells top off in capacity. Nominal is what is advertised - when someone says 7.4v 2S lipo they are talking about the nominal voltage even though when fully charged, that 2S will indicate 8.4volts (4.2v/cell x 2 cells).

The C business is the batt capacity divided by 1000, so for a 5000mah lipo, its "C" is 5amps (5000/1000). This is used to specify charge rate as in a 1C or 2C charge (for same 5000mah lipo, a 1C charge is 5amps and a 2C would be 10amps). The other way the C rating is used is for discharge capability. A 5000mah lipo with a 20C rating can discharge 5 x 20 or 100amps contniuously. Lipos are usually given continuous and burst ratings. These numbers should be based on testing, but since there is no industry standards, some 25C rating is another manufacturer's 20C rating, these can be inflated sometimes. A little like advertised nitro horsepower numbers.

Lipo are made using 2 or more individual cells. the S number is the number of cells wired in series - as in 4S - thats 4 3.7v cells assembled in series for 4 x 3.7 = 14.8volts nominal. Fully charged it would be 16.8volts. The 2nd number you see sometimes is the # of cells in parallel as in 4S1P or 4S2P. Adding cells in parallel adds capacity, but does not increase voltage. If a 4S1P was 5000mah capacity, a 4S2P would have same voltage, but 10,000mah capacity. Adding cells in parallel is usually done to increase run time.

Since lipo has more than one cell, the balance of cell voltages is important. If the cells are out of balance and you charge it, there is potential to overcharge the cell with highest starting voltage. Chargers look at total voltage and not individual cell voltages (there are chargers that charge cell by cell, but they are not common). Same for discharging or running with a lipo. A low voltage cutoff (lvc) looks at a total voltage like 6.2v so if the cells start off out of balance, one cell can be overdischarged and the lvc would allow it since combined voltage could still be more than the lvc. Balancing cells with a balancer minimizes possiblities of either overcharging or overdischarging cells.

Some chargers do not have builtin balancers, some do. Those that don't can use a separate balancer. Separate balancers can perform charge thru or standalone balancing. Better ones will do both. Charge thru will give you a little more complete charge since standalone balancing simply checks cell voltage and discharges the rest of the cells down to that lowest voltage. A charger with balancing capabilities does the balancing while charging getting all the cells as close to cell max voltage as possible.

Lipo cells vary in quality and cost. The most expensive cells are produced with rigorous QC and carefully matched with others on assembly. Cheaper lipos don't take these added steps. Cheaper lipos get out of balance easier and need more in the way of balancing than the high quality lipo. Even the best lipos eventually do need to be balanced to maximize life and performance.
Wow! That's a lot to process. I'm going to have to go over that a few times to make sure I take everything from it... But major thanks.
I should've known I was going to wind up with just as many new questions as answers. Ill get to those a little later.
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Old 04-12-2009, 07:14 AM   #6
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Default Lipo cut off

also looking to make the change to lipo can anyone recommend a good cut off until I change out my esc?
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