I am looking for some help with wiring for a large battery array using 18650 lithium ion batteries.

Ok so I am looking at going solar and lead-acid batteries are too low overall capacity and their life span is too short.

I would like to make a number of batteries using 18650 wired in parallel to give me overall a 12v or 24v (depending on charge controller) 2500Ah or 1250Ah depending.

I am thinking I can in a 12v system wire 4 18650 batteries in series, that will be my cell and then say 100 cells in parallel and call that a block. I can take 4 of these blocks to give me a large battery with enough capacity for running my house and using a solar array to charge them via the charge controller. I want to know if this is practical and what are the balancing issues? Each 18650 battery is 3.7v 4900mAh. By my calculations and please explain where I am going wrong Cell: 12v * 4.9 = 58.8Wh Block: 12v * 490a = 5880Wh 4 Blocks: 5880Wh*4 = 23520Wh

The Batteries I can get at around $1.3 USD a piece. I am aware of the need for balancing and am concerned about safety issues.

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    \$\begingroup\$ I'm not aware of any reputable battery manufacturers who sell 18650 cells with 4900 mAH rating. That said - I'd very much like to be wrong about that. Can you tell us who makes those cells and where we can purchase them? \$\endgroup\$ Commented Apr 24, 2015 at 21:28
  • \$\begingroup\$ industrial.panasonic.com/lecs/www-data/pdf2/ACA4000/… seems a semi-decent datasheet. From that, you probably want to avoid discharge below 3.2V, which suggests you'll get 2.5Ah at ~3.7V average, or 14.8V * 2.5Ah = 37Wh per 4S cell. \$\endgroup\$
    – CoderTao
    Commented Apr 24, 2015 at 21:39
  • \$\begingroup\$ No, Dwayne, you are not wrong, see my answers note. \$\endgroup\$
    – Overmind
    Commented Jun 28, 2016 at 7:38

4 Answers 4


Advice: 1) Your battery bank should be 48V (otherwise wire diameters will become unreasonable). Run through the calculations and you will see what I mean. For example, if you need, even momentarily, 50 Amps of 120V AC, that would be over 500 Amps at 12V.

2) Your charge controller must be designed for lithium batteries (or be programmable). Lithium batteries cannot be safely floated. You must stop charging them once charge is complete.

3) Investigate any building code issues prior to purchasing anything. If you are going to pull permits to do this, you will need approved plans. An inspector might not allow you to put in a large lithium ion battery pack without UL approvals or some such. If you live in the US you should be concerned about this. If you are outside the US, you are outside my experience.

4) When you put all those batteries together and charge or discharge them rapidly, the batteries in the middle will be unable to dissipate heat effectively. I don't know how big a problem this is but I would be very worried about it. I think you will need a thermal sensor near the middle of the pack to make sure the batteries do not overheat.

5) You cannot put the batteries in parallel without some type of over-current protection. It could be as simple as a fuse or PTC. The reason is, you don't want one bad cell (imagine it failing short circuit) to become a sink for all the other cells in parallel. This could make a bad situation much worse.

That is all I can think of at the moment. I can tell you right now that I would not do this. I would use AGM lead acid batteries. If you provide individual over voltage protection for each cell, that might help prevent the dangers of imbalance. When one cell in a series string stops accepting current, the whole series string stops. So the string would only be as good as the weakest cell.


To start, Dwayne Reid has an excellent point, an 18650 with 4900 mAh capacity is incredible, and at $1.30 per battery it is truly an amazing deal. If it is legitimate, then there are a lot of people who would like to know where you are getting these 18650 batteries. So there is the very real possibility that the company selling these batteries is over estimating the capacity and if this is the case, then the safety of these batteries will be suspect as well - be careful!

Having said that, your question is still a good one.

When using the 3.7 volt nominal 18650 cells, typically 3 are used in series to get a nominal 11.1 volts and 12.6 volts fully charged. There are some solar charge controllers that use 4 - 18650 for a nominal 14.8 and a fully charged 16.8 volts. Which one you use will depend on your solar charge controller and then your inverter (if you use one), and the voltage that your appliances run on if you don't use an inverter.

Balancing issues: Over time, with repeated charges and discharges, it is likely that one or more of the 3 cells in series that make up the 12 volts (the 3 cells in series might be 3 'blocks' of 100 - 18650 batteries first conneced in parallel then wired in series) will become unbalanced. This means that one or more of the cells are of different voltages during the charge and discharge. This usually happens because of different internal resistance between the different cells. The charger will charge the battery to 12.6 volts, but one cell might be 4.2, one cell 4.1, and one cell is 4.3. The total voltage is normal, but the individual voltages are unbalanced and this can be harmful to the individual cells and even dangerous, especially to the overcharged cell. A balance charger will monitor the individual voltages of each cell while charging and ensure that at the end of the charge, all voltages are the same.

Calculations: To be accurate, you should use the nominal voltage which for the 18650 is 3.7 volts per cell or 11.1 volts for the 3 cell 12 volt battery. Also, assuming that you are using 100 of the 18650 cells and they are 4900 mAh.

4.9 Ah x 100 = 490 Ah per 100 - 3.7 volt 'block'.

3.7 x 3 cells = 11.1 volts. 11.1 volts x 490 Ah = 5439 Wh.

Hope this helps!

Please don't forget, 100 - 18650 cells in parallel is a lot of energy and you could certainly cause a lot of damage if it short circuited.

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    \$\begingroup\$ Just an update on this topic. I asked the sales agent if they can supply a certificate of compliance with my countries electrical standards but it appears they can't so I very much doubt they are reliable in their stated capacities (4900mAh and 7800mAh) I have just recently seen a video of a 18650 exploding and it is shocking to say the least. I have been looking a more reputable dealer and I can purchase Samsung 18650 batteries for about $2.5 for a 2600mAh battery. I realise it is a lot of power, these all have individual PCMs so that might help. \$\endgroup\$
    – Psyonus
    Commented Apr 26, 2015 at 9:07
  • \$\begingroup\$ I have have been thinking if I make my battery using a 48V 30A BMS device so the arrangement would be 13 series, 12 parallel totalling 156 batteries. Each 18650 has its own PCM and each parallel block is attached to the BMS device and the setup has fans with thermal sensors. I am looking to have 30KWatts of storage. Is this enough safety procautions or should I still abandon the idea? \$\endgroup\$
    – Psyonus
    Commented Apr 26, 2015 at 9:32
  • \$\begingroup\$ The BMS certainly adds a level of safety that most people find appropriate. Each cell having its own PCM also adds safety. The one concern is the very rare possibility that an individual 18650 shorts to open and then the remaining 11 in the parallel bank dump all mod their power into that cell, since it has essentially become a piece of metal conducting the entire parallel bank. I would ask Samsung if their PCM protects against this - I believe it does, The e bike people who use 18650 tend to ignore this risk and so far I have not heard of any problems, so I think it is a rare possibility. \$\endgroup\$
    – Filek
    Commented Apr 27, 2015 at 2:24
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    \$\begingroup\$ The only people that I have ever heard that actually protect against the possibility of an individual cell shorting to open is the Tesla battery pack. They protect against this by adding a fuse that will blow if any cell tries to feed a current greater than say about 30 amps, OR if the entire parallel bank tried to pump 30+ amps into the one fried cell, the fuse pops and the connection is disconnected and no current flows. They don't use an actual fuse, just a thin wire that melts away when the current is too high, it is just too much labor for the average person to build a battery that way \$\endgroup\$
    – Filek
    Commented Apr 27, 2015 at 2:29
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    \$\begingroup\$ By the way 13 series and 12 parallel is only 1.4 KWH of storage. If you follow standard charging and discharging guidleines, especially not overcharging, it goes a long way to keeping your battery pack safe. Not overheating and keeping the current you draw within the battery's limits all help keep the battery from exploding - which is rare for a reputable manufacturer. The BMS usually prevents you from over stressing or abusing the battery anyway. \$\endgroup\$
    – Filek
    Commented Apr 27, 2015 at 2:37

I have built several large battery packs for ebikes.. As far as I understand it, there are several ways to go with this:

  1. Use prismatic LiFePO4 cells, like thundersky, Sinopoly, CALB or other brand. They are dead-simple to use with bolt connections. For example: http://www.ev-power.eu/Sinopoly-40Ah-300Ah/ Only problem is energy density - only ~100 Wh/kg.
  2. Use 18650 cells with chinese brackets. For example: http://www.aliexpress.com/item/Free-shipping-18650-battery-holder-Cylindrical-battery-bracket-18650-li-ion-cell-holder-18650-batteries-fixture/32304025030.html I have used these; assembly is very simple and you can do batteries of all kinds of weird shapes. But this raises concerns with many cells in parallel...
  3. Use prebuilt 18650 modules like this: https://www.energusps.com/shop/product/li-ion-building-block-3-6v-78ah-12 they advertise these as being "safe": with fuses on each cell and even vent channels. They also have 20Ah version that is apparently used by electric formula teams. So must be decent quality and Wh/kg ratio. Bit pricey, though.

Also many DIYers do "batteries" from RC LiPOs, but after seeing all of problems they are having, included many burned down houses, I would not go that route.


I would not really recommend such an approach today given the lower quiality of Li-Ion Cells. Form heavy testing, it seems the 2200 mAh Sony and Samsung cells were the last one I could say have very good quality. The newer models just don't last much. What you try to accomplish is actually what the so-called electrical cars are using: big Li-Ion packs. The difference is that their cells are specially designed for this.

Anyway...if you decide to take this approach, you may want to consider the following:

  • For 12V compatibility you will need 3 cells in series, not 4. At full charge, a standard Li-Ion cell has 4.2v, that will make the total of 12.6 at full capacity, which will drop when discharged. At exactly 10V total you may consider the bank empty and recharge.

  • The high number of cell you will use will need some type of removable connectivity. This is important because cells may be damaged and will need easy replacement. A spring-based block where you can remove any cell independently should do in this case.

  • Connected to the above, it is the monitoring that may be a little more difficult. There are cheap indicator LED circuits that monitor up to 8 cells and having those installed on every cell can make sure you can quickly eliminate bad cells, but it will increase the overall cost of your project.

  • The problem with multi-charging is that in-series cells tend not to charge identically across all cells. For example, in a 3-series pack, cell one may charge 100%, cell 2 95% and cell 3 80%. That will cause the first cell to eventually take overload damage, the last one to slowly die and the mid one to be inefficient. This is a well visible problem with laptop batteries today - that's why they die so fast. Therefore, it would be better to devise a circuit that charges parallel packs or even each cell independently. That will significantly increase costs and complexity. Of course, you can bet on cells being good enough to last long enough, but the probability of that decreases a lot when you have many cells. There are circuits called BMS for balancing series charging.

  • Safety concerns are usually related to charging. Bad cells will overheat and can never reach charge level, but can overheat enough to cause a fire. Also, the charging circuit for big banks would have to be well calibrated and have components good enough to withstand the very high current.

  • The charging is also an issue. For big packs you will need very high current and the ability to detect malfunctioning cells while charging (can be detected temperature-based - bad cell will tend to overheat when charging at corresponding current).

  • From a capacity perspective, if you want to obtain 2200 AH out of 2200mAh Li-Ion (or lets say 2500AH out of 2500mAh cells) you will need 3000 cells (something like 3 packs put in series out of 1000 cells for each in parallel). That will give you 30KWh+ (37KWh ideal top value) from a power perspective.

IMPORANT NOTE: there is no 18650 with 4900mAh capacity. Those are fakes. All Ultrafire cells currently labeled above 3000mAh are utterly fake and have a practical typical capacity of 500-800mAh. The super-fake ones are at 400-500mAh (they have a smaller cell inside and sand to fill-in) and the 800mAh ones are recycled relabeled cells. The easiest way to detect super-fakes is by weight. An original Li-Ion has over 45 grams while fakes have under 30.

Currently, the highest capacity cells are at around 3400mAh and are being produced by the big companies: LG, Sanyo, Sony, Samsung, Panasonic.

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    \$\begingroup\$ Your last note about spotting fake 18650 cells is really helpful since I've seen 1,200mAh-9,900mAh ratings. Unfortunately, online sellers (even reputable ones) don't always list the cell weights. \$\endgroup\$
    – Xeoncross
    Commented Dec 31, 2016 at 4:13
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    \$\begingroup\$ I prefer recovering cells from other things like notebook batteries instead of online purchases. In most situations, in any bad notebook battery at least one pair is good (as in 90%+ capacity) with at least one more recoverable, while one is most certainly bad (irecoverable). In case of need of new ones, I only buy official manufacturer cells. \$\endgroup\$
    – Overmind
    Commented Jan 6, 2017 at 11:48

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