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Inexpensive reserve battery

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Incidentally it is almost always better to parallel equal voltage packs, of the same Ah, same battery manufacture and type and age, than switch between them. This reduces the current load on each and makes each more efficient, and delivers a higher voltage to the controller under load by reducing the IR Each battery will deliver more amp hours at the lower current as well. 

Edited by Tabletteer
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  • 4 months later...
On 7/2/2016 at 2:57 AM, william said:

I put 2 in a small bag for almost double the range.



On the thin there isn't room in the box unless I take out the stock battery. I also used velcro.



Can you give a follow up on

1. exactly how you wired these 2 reserve batteries, charge them and  and connected them when using 

2. How this 'experiment' is working so far for a Sondors thin reserve battery pack, any issues, reserve battery range, etc

3.Link to where you bought them and other connectors. Any issues in getting them delivered ( I've heard lots of stories about hassles wrt delivery on Li Ion batteries?

Thanks !!

Edited by Boulderite
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Good background information about connecting (paralleling) packs from electricbike.com. 


Paralleling BMS protected Li-ion packs. Short Answer

It's getting harder and harder to fit normal packs on to full suspensions bikes with small triangles and with the new small mini cubes that Luna offers are a godsend....but maybe one is not enough?

Or you just want to get more range without having a big triangle pack so paralleling has been a question that is coming more and more frequent.

So what is paralleling 
battery pack? it's taking 2 packs and connecting the red positive lead to the red and the black negative to the black, the output voltage stays the same but the maximum current capabilities are doubled and so is the pack capacity!

Here is a basic diagram of 2 "AA" batteries in parallel to explain it simply:

As you can see , volts remains the same and capacity doubled.

Now can we do this on our Li-ion 18650 packs?

To answer this, the very same pack you have is in fact a bunch of single cells put in parallel then these parallel string are put togeter in series.

Let's take the famous 14ah Shark 52V Panasonic GA pack for example: it's a 14S4P pack, it's 4 cell in parallel (4P) giving 3.7V and 14ah, then 14 of these strings are put in series making it 52V 14ah. Then a BMS is mounted on the pack to protect it against overcharge, over current, under voltage, high temperature, short, reverse voltage, unbalance and etc...

Now can we parallel 2 complete packs together and get more range and more current?

Simply put yes! If both packs are new and they are identical (same voltage, same cell model, and same number of parallel strings "P") you can plug them in parallel and they would just equally share the current draw.

Ex: 2X 14ah shark packs, they both have 30amps continuous rated BMS, tighter they make a 52V 28ah pack. So if lets say you run them at 30amps for the whole trip (quite impossible but it's an example) both pack would share equally the load, so they would both give out 15amps of current till they hit the cutout from the controller. After this trip if you then unplug them completely they should both have roughly the same low voltage as they had the same current draw. This is the ideal solution.

The what if's? (Q & A)

Q: What happens if they have not same voltage when I connect them together? 
A: The stronger pack will try to charge the weaker pack through the discharge port till the voltage is equal on both packs. depending on the voltage difference the current can be really high.

Q: Isn't it good that they charge themselves like that?
A: NO, depending on the type of cell you have (the internal resistance), the voltage imbalance between the packs and the type of BMS you have can result in problems. The internal resistance will dictate how much current can flow between the 2 packs depending on the voltage difference, and can easily go over the BMS limit for reverse voltage (remember you are charging through the discharge port). This can result in BMS damage or cell damage.

Q: What to expect in terms of cross current if I can only get the 2 packs to about 1V close.
A: Like I said it depends on the internal resistance of the pack, but it's kind of an advanced question. Lets keep it simple with a clear example:

I'm personally running 2 Luna Mini cubes 6ah 52V with Samsung 30Q's, I recharged one to 58.4V and the other to 57.4V. Will it blow the BMS, will I see massive current?

Cycle analyst on one pack and batt man on the second pack, (both are very precise compared to a professional multimeter)


To measure the current between the 2, I made an adapter to match the female XT-90 on the batt so I could use the Cycle analyst shunt in between the packs. The packs are not connected to the motor. The Cycle Analyst has a Amax feature and records the highest current spike crossing the shunt. And with just 1 volts of difference I measured 1.79Amps


After a few seconds the current was already below 1amps and after a few minutes the current was below 0.1amps and the voltage is settling at 57.7V

Q: So is it safe to charge VIA the discharge port at 1.7amps?
A: Yes it is, direct drive hub motors have regen braking and can send current higher than that to the battery. So It's safe for a small current.

Q: Can I use a large pack and a small pack if they are at the same voltage?
A: Short answer for inexperienced users, No! 

Longer answer :Connecting two banks with different capacities in parallel is technically fine since the batteries will be operating at the same voltage. Charge and discharge current will be shared, based on capacity. It is best if the batteries are of the same type and age.

But different packs have different internal resistances. The Ohm's Law tells us that the stronger battery will discharge through the weaker battery -- you will always have current flowing

So do I recommend it? Not really unless you know exactly what you are doing, this is my advice and you are welcome to share your experience with me...and real life test results too!

Connectors-R-Us   https://hobbyking.com/en_us/catalogsearch/result/?q=XT+90+connectors

Edited by Tabletteer
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