I am by no means an expert on the subject of LiPos, however I do have a tiny bit of experience with which to share. In brief, we can study the problem as follows:
Battery Chemistry:
Before selecting I made a quick-comparison of different chemistries and their merits; LiPo, LiFePO4, NiCd, etc. For my money - LiPo has the highest power density at the best price-point by virtue of mass-production for the RC market and by access to inexpensive suppliers - such as HobbyKing. Once you have sources to a good consistent supply of batteries then we evaluate what can accomplish with the available stock.
Distance:
Next, evaluated how far we need to go for daily commuting (or other significant goal). It is a good idea to pad this figure by 20% for peace of mind. I had a parallel goal of long-distance cross-country travel, so whatever battery I chose – it would have to serve in both roles, and it had to be reliable.
Voltage:
It is important to understand what voltage you ultimately need to have. Most people start out using 36 or 48 V controllers which is very common. I already had a 36V controller that was part of a kit and came with a LiFePO4 battery pack, therefore upgrading to 37V (42-30V LiPo range) was not an issue.
Cell Count:
It so happens that 37 / 2 = 18.5 => 18.5 / 5 = 3.7 V …or the nominal voltage of the LiPo cell. Therefore I chose 5S1P as my “base unit configuration†and stringing 2 of these together in series gets me to 37 V. Easy math.
Generally, the voltage matrix goes as follows for common configurations:
Code:
Volts / Cell-Count / Qty units required in series
37 / 10 / 2 * 5S
44.4 / 12 / 2 * 6S
44.4 / 12 / 3 * 4S
55.5 / 15 / 3 * 5S
59.2 / 16 / 2 * 8S
59.2 / 16 / 4 * 4S
66.7 / 18 / 3 * 6S
74 / 20 / 4 * 5S
74 / 20 / 5 * 4S
Amp-Hours:
Now we go back to the issue of distance required. I need to travel about 22 miles/day minimum; let’s pad this up to 30 miles. Through previous testing 1 Ah @ 37 V will take me 2 miles in ideal conditions. Therefore 30 miles / 2 = 15 Ah.
C-Rating:
This is the value at which rate the battery can discharge or charge. It is typically denoted for example as 15/2 or 20/2. The first part of the figure means that the battery can discharge at 15X or 20X the Ah value in one hour, and the second part means how fast the battery can recharge, at 2X in one hour. The typical discharge range is 15 to 45. For most applications 15 to 20C is plenty good, with higher ratings costing quite a bit more.
Find a Match:
I have experience with HobbyKing (HK), and they sell a 5S1P battery in 5000 mAh (5 Ah) for ~$40 and it weighs 1.27 lbs each. If I need 15 Ah minimum, 15 / 5 = 3 units in parallel. We already calculated that 2 units were needed in series to create 37 V, therefore the complete battery assembly is (2 * 5S) * 3 … which is written as 10S3P, or 6 * 5S1P. The weight would be 6 * 1.27 = 7.62 lbs. The cost is 6 * $40 = $240 USD.
Pop Quiz:
What other unit configurations are possible for 37 V given 4S, 5S, 6S, and 8S units?
- A: 4S + 6S = 10S in series - which would become the Base Unit to parallel for increasing Ah.
More-Advanced Analysis:
Now if you really want to get tricky, I created a Spreadsheet that tracks
- Brand
Ah
Cell-count
Total Voltage
C-Rating
Weight in Grams
Unit Cost
Dimensions
Supplier
From this I extract:
- Cost/Cell
Cost * gm
Cost/Watt
Watt/Weight
Cost/Ah/Cell
Weight in Pounds
Total Unit Watts
Ah / C rating
L, W, H
Once the table is built I populate it with real data from suppliers, and apply conditional formatting with color scales:
Green = Optimum (target),
Yellow = Average (median),
Red = Extreme (edge-case).
The battery with the most Green wins. In truth – there are no clear winners; you must decide which features are most important. I chose by maximum power density: Most voltage and Ah per gram for a given price - which may vary from month to month. I also chose alternate battery packages in case my primary choice was unavailable or discontinued. Caveat: Generally there is value in selecting one base unit and staying with that brand; don’t mix chemistries or configurations, and Keep It Simple Silly (K.I.S.S.)
Conclusion:
Sorting out batteries in this manner can be fun; it’s like a puzzle – with each piece having something to offer. My advice is: Take your time, be picky, ask questions, chose wisely.
I did not use a BMS for many months; however I have been diligent about keeping batteries in balance. I would not mix a BMS conversation in this thread other than point to references on the subject.
I built a custom-charger assembly to suit my needs out of modified Meanwell units. Again, I’d defer to other threads on the specific subject.
Best of luck,
KF
