dogman dan said:
To get to 72v, you need 24v 20 amps of something. It doesn't have to be lifepo4, hell, it could even be lead if you happen to have some.
"...24v 20amps of something..."
?
For a series solution, the idea is to boost the available power by upping the voltage (V x controller_rating). Here you can use your 25A controller or the 60A controller with CA limiting to 30A.
You will need 24V with a 20Ah rating and a 25A or 30A continuous discharge rate (depending on the controller). If you get less than 20Ah, the booster pack will drain before the 48V pack, making part of the big pack unusable as the booster pack BMS cuts out. With less than 30A continuous discharge capacity for the booster pack (the assumed continuous rate of the existing pack), the discharge rate will be limited by the smaller pack reducing the current you can draw even lower than what you already have.
- Realistically, you might use a slightly lower Ah rating for the booster pack, leave part of the big pack unused, and still end up ahead, but it seems a questionable choice. Looking at capacity (affects range) in Wh and power (climbing ability) in W:
Code:
Presently: 16cells x 3.2V/cell x 20Ah = 1024Wh, 16cells x 3.2V/cell x 25A = 1280W
Adding a 15Ah 24V pack: 24cells x 3.2V/cell x 15Ah = 1152Wh, 24cells x 3.2V/cell x 30A = 2304W
Adding a 20Ah 24V pack: 24cells x 3.2V/cell x 20Ah = 1536Wh, 24cells x 3.2V/cell x 30A = 2304W
Clearly the 'new' extra current expended climbing is going to eat into your Wh (range), but you can see the tradeoff...
Again, you will need to mess with the simulator to see what these numbers mean in terms of climbing speed and heating.
In this serial case it's not necessary to use the same chemistry. Ideally, the booster pack has a BMS or is manually monitored, since the CA pack-level LVC cannot accurately protect series packs with cells that are dissimilar (capacity, age, chemistry, etc).
In the parallel case:
dogman dan said:
My best advice, buy the 48v 40 amps controller, and some more 48v battery.
...
You could also add 5 ah of turnigy, adding 14s 5 ah would stiffen your amps limit. three 4s packs, plus one 2s. two 4s plus two 3s. Whatever adds up to 14 in series.
This is a bad idea. You really don't want to mix LiPo and LiFePo4 chemistries for parallel packs. Although the 'fully charged' voltages are very similar, the discharge curves and termination voltages are very different - since LiPo termination voltage is much higher than that of LiFePo4, you will need to stop discharge while much of the LiFePo4 capacity remains unused. Here you would need to be sure to use the CA LVC set to the LiPo LVC since presumably the LiPo has no BMS.
From this thread (https://endless-sphere.com/forums/viewtopic.php?p=988017#p988017):
dogman dan said:
Lets get into the details anyway. When I combined 14s lipo with 16s lifepo4, it was pretty awkward. They did start at about the same voltage, 58v.
But on the other end, I had to stop the lipo while there was still a fair bit of capacity in the lifepo4, or risk over discharging the lipo.
If you are going to address your issue with a parallel booster pack, stick with the same LiFePo4 chemistry.
In either the serial or parallel cases there are certainly ways to reconnect packs during the journey to manually manage incompatible battery characteristics, but IMHO these are academic or make-do exercises and not the way you want to plan a new battery purchase. Get the proper chemistries, capacities, and discharge rates and either approach should be trouble-free - otherwise you will have a PITA for the life of your batteries. In any case, read some threads about charging/connecting serial/parallel packs.
