Making my own Li-Ion/Po battery pack - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: April 29, 2014, 03:35:46 pm
content: There are no suppliers of lithium battery packs in the UK, except for golf batteries which are quite expensive, or the company Tracer which is also quite expensive. I have not found anything likehttp://www.batteryspace.com/here in the UK.So I have decided to make my own 12V battery pack at around 10Ah-12Ah. The equivalent Tracer costs £150-£180.I intend to buy a few of these from ebayhttp://www.ebay.co.uk/itm/KDQ4-EU-plug-9800mAh-DC-12V-Super-Rechargeable-Lithium-ion-Battery-SPC-2261-/331039702802?pt=UK_ConsumerElectronics_Batteries_SM&hash=item4d1380ff12I will open them up and remove all the outside packaging which is around 170grams leaving just the 3 cells which are also 170 grams altogether. I plan to remove the individual cell PCBs and instead use one of these for each 3 cells:http://www.ebay.co.uk/itm/321058088538?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AITThese 3 cells in a mini-pack will have a capacity of around 3000mAh (down to 10.8V or 3.6V per cell).I will then take 4 of those mini-packs and put them together in parallel. Each minipack has its own PCB as above. The total pack will be 4 * 3000mAh = 12Ah. With all 4 mini-packs wrapped up in one, it would weigh around 800 grams.The total cost of the pack would be around £90 ( around $135) almost half the price of the Tracer.The only glitch is that these batteries come from ebay China, there is no warranty and they could be faulty.Is this a good idea ?

author: jamesglanville
author_rank: Contributor
timestamp: April 29, 2014, 03:57:33 pm
content: I would personally be tempted to get some RC battery packs. Hobbyking stuff isn't insanely good, but their stuff is more reputable than a random ebay seller IMHO. A couple of these:http://www.hobbyking.com/hobbyking/store/__37359__ZIPPY_Traxxas_8400mAh_3S1P_30C_Lipo_Pack_Suits_TRA2878_.htmlmight be a good choice. At $64 each for 8.4Ah that's pretty cheap. You'd only need to parallel two sets of cells as well, which would make it easier.Do you only need to draw <4A? (that's all the pcb you linked is rated for). Also the cheap voltage checkers often have a sizeable leakage current in the tens of mA for the crappiest ones, which can be annoying.

author: akis
author_rank: Frequent Contributor
timestamp: April 29, 2014, 05:56:33 pm
content: Oh thanks for the link, this is very interesting.I am looking to order these:http://www.hobbyking.com/hobbyking/store/__34884__Turnigy_nano_tech_8400mah_3S_40_80C_Lipo_Pack_TRA2878_Slash_VXL_Slash_4x4_compatible_UK_.htmlNow trying to establish the exact specs.

author: staxquad
author_rank: Frequent Contributor
timestamp: April 29, 2014, 06:03:09 pm
content: specs on that PCB are kind of useless for LiPoMaximum protection voltage ( for every single cell ): 4.255V ± 0.05VMinimum protection voltage ( for every single cell ): 2.35V ± 0.05Vmax 4.1V desirable, at very max 4.2V, 4.255V will shorten life of cellsmin 3.3V desirable, no power any lower, 2.35V will damage cellsThat Ebay LiPo doesn't have balancing wires, will not have a PCB and nowhere do you mention balancing those cells in the new packs with a balance charger. LiPo cells need to be kept in balance, otherwise some go hi and some go low from the charging/discharging cycling, the charger is charging the aggregate cells to 12v, not charging each cell to 4.00V (4.1V, 4.2V), so doesn't detect low cells and high cells adding up to 12V, the high cells damaging in charging and the low cells damaging in discharge.I'm not understanding your assembly. If you've got 3 cells in series in a 9.8Ah pack with a PCB and you put 4 of these packs in parallel, you'll have a 12V 39Ah mega pack.Manufacturers also match each cell impedance when they assemble packs.Select battery packs from HobbyKing that match your requirements, they will come with balancing wires, get a good balancing charger that displays cell impedance.

author: mark03
author_rank: Frequent Contributor
timestamp: April 29, 2014, 06:17:23 pm
content: What sort of peak currents do you need? I am assuming "portable electronics" level (about an amp, or less). If that is not the case, disregard the following.I stated in another thread (or two) why I think hobby RC batteries are a bad idea. They are optimized for high currents rather than cycle life, and the high current capacity plus the lack of a built-in protection circuit makes them a lot more hazardous. (BTW the LiFe suggestion in that other thread was a good one!) You can always add the protection PCB yourself, but stop for a moment to consider what happens if you slip and the wrong wires contact... at least do this operation outside so you can run if things head south.Finally, about the battery from China, personally, I would be VERY cautious when it comes to Li-ion cells of unknown provenance. Of course, it is hard to avoid buying from China, in which case have you considered going with 18650-size cylindrical cells? Lots of guys in the LED light hobby are testing these and sharing the good and bad vendors, so at least you can average the reviews. I got some very nice LG cells from fasttech.com and can tentatively recommend them.

author: akis
author_rank: Frequent Contributor
timestamp: April 29, 2014, 06:32:42 pm
content: The ebay "12V 9800mAh" packs ARE NOT "9800mAh" - they are around 3000mAh. They contain 3 cells of 3000mAh each, in series, to give 12.6V fully charged. Therefore I need four of these ebay batteries to make a REAL 12Ah.That PCB/PCM from ebay is designed for 3 cells, and protects them independently. I am not sure if it will balance while charging, or after charging. It is right that the "limits" are way too high, 2.5V is too low. But at least it is better than nothing.My portable device will operate between 12.5V and 11.0V. I do not plan to take the batteries any lower. The charger will charge at 12.5V. The current draw is a constant 1.3-1.4A without any spikes.I was not aware that those hobbyking batteries come completely unprotected without a, I had thought all commercial LiOn batteries come with some sort of PCB/PCM! I just chatted online with a representative and he confirmed it. Damn. Maybe I can then add that ebay PCM so as to protect against accidental shorts, over discharging etc?Edit: apparently the battery has a separate connector for balancing. Even if it did not, we can always take it apart and charge the cells independently.

author: Neverther
author_rank: Regular Contributor
timestamp: April 29, 2014, 06:37:52 pm
content: staxquad: Those batterypacks are fake, the Ah rating on the title has nothing to do with reality.My guess is that they are not even 3000mAh.I've done packs from protected cells, 2400mAh batteries (ultrafires from DX, on one order they sent me different SKU with chinglish text and the protection board shorted itself on one cell) 6 of them comes around 25usd, gives 3S2P config with about 4.2Ah.So three of those comes around 75usd which is around 45gbp for about 12Ah.Built in PCBs on each cell, casing built out of plastic and tape with 2.1mm plug for power and JST-XH for balancing.5 year old pack still reads 2700mAh so you get what you pay for, all batteries will die eventually.

author: staxquad
author_rank: Frequent Contributor
timestamp: April 29, 2014, 11:12:18 pm
content: Quote from: Neverther on April 29, 2014, 06:37:52 pmI'd avoid them like the plague, will only disappoint. They are larger than their capacity, so are a waste of space and weight, and lose their charge in a matter of days, when new lithiums keep their charge for months, and their voltage drops like a rock upon loading, their wattage output will be lower from the get go, totally useless.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 30, 2014, 08:19:10 am
content: if you buy batteries from hobbyking, i made this a while ago, it should give you an approximate idea of $/watthour and which are the best value

author: Psi
author_rank: Super Contributor
timestamp: April 30, 2014, 08:40:40 am
content: The hobbyking nano-tech stuff is really good.Especially the high-C red colored nano-techs.The generic brand turnagy stuff is ok too, i measured some of their cheap turnagy 3Ah 20C packs (the blue light ones) and got a real-world capacity of 2.6Ah at i think 1amp discharge.

author: akis
author_rank: Frequent Contributor
timestamp: April 30, 2014, 09:19:10 am
content: I bought two of thesehttp://www.hobbyking.com/hobbyking/store/__34884__Turnigy_nano_tech_8400mah_3S_40_80C_Lipo_Pack_TRA2878_Slash_VXL_Slash_4x4_compatible_EU_warehouse_.htmlI was told they are 12.6V fully charged. Not to go below 9.9V which is 80% of their capacity spent (6720mAh).

author: akis
author_rank: Frequent Contributor
timestamp: April 30, 2014, 09:44:33 am
content: What is the purpose of the "balance charger" connectors available on the li-on chargers? Are there for monitoring purposes or for charging purposes? Is it not more straight forward to charge each cell individually through an isolated DC charger? That would ensure that each cell gets the right voltage and the right current, and that no cell is reversed biased or subjected to higher currents than normal.

author: Psi
author_rank: Super Contributor
timestamp: April 30, 2014, 11:02:07 am
content: It's for both.A balance charger will charge the cells through the balance connection then stop for a sec and check the cell voltages before charging again.If it detects one cell is full then it stops charging that one but continues to charge the rest.It's a bit more complex than that but yeah, that's basically it.

author: Kompost
author_rank: Contributor
timestamp: April 30, 2014, 01:00:11 pm
content: Quote from: Psi on April 30, 2014, 11:02:07 amDepends on the charger, cheap hobby ones only charge through the main connector and discharge through balance taps. Similar to an independent cell balancer. They have to discharge the cell that went too high a bit before continuing charge, that's why they sometimes take ridiculous amount of time to equalize cells.

author: akis
author_rank: Frequent Contributor
timestamp: April 30, 2014, 03:14:39 pm
content: Why not charge the cells individually and not have to worry about balancing half way through, stops and starts, one weak cell drawing more current than others etc?In other words, instead of charging at 12V and then hoping for the best, why not charge 3 * 3.6V from isolated chargers? (or whatever the actual voltages are)

author: staxquad
author_rank: Frequent Contributor
timestamp: May 01, 2014, 02:45:09 am
content: Quote from: Kompost on April 30, 2014, 01:00:11 pmThere's a good reason for that, the balancing cables are AWG 24 Gauge, and healthy cells are all reasonably matched, so charge equally anyway. You have to have cell mismatches to worry about balancing large differences. A good balance charger will handle charging any healthy matched cell battery pack with ease.I use a 250 Watt charger (iCharger 106B) and charge my larger ZIPPY Flightmax 8000mAh 3S1P (and 2s1p) 30C batteries at 5 amps. Doubt you'd want to charge through 24AWG gauge balancing cables with that current. And it doesn't matter, when cells are healthy, their impedance match, so they charge and discharge pretty even.I do have a mismatched battery that I charge when I need it, so plug it in, check where it's at, charge it, draw down the high voltage with a little connector and a bleed resistor on the high voltage cell if I have to (not always, depending on circumstances), not too fast, manually check, once pretty even it gets charged to completion. When you know the health of the cells, charging ain't a mystery.A more upscale system isn't really required for hobbyists.akis: Why not charge the cells individually and not have to worry about balancing half way through, stops and starts, one weak cell drawing more current than others etc?It should be simple, fast and efficient, charge the battery, use the battery, keep the health of each cell in the battery to maintain output power and longevity, monitor each cell in the battery to know what's going on, no screwing around with assembling and disassembling connections and possibly making mistakes and having a multitude of chargers for each cell or having one charger and charging each cell serially, needing lots of manipulation and time.

author: akis
author_rank: Frequent Contributor
timestamp: May 01, 2014, 07:46:20 am
content: I see what you mean, 24AWG is too thin to take any amps.If I were building up a pack, I'd expose the individual cells with thick wire and hefty connectors so that they can be charged individually. I would built a charger that has 3, 4 or 5 outputs at 3.6/4.2V rather than one 12V output and 5 "balance " ones.

author: peter.mitchell
author_rank: Super Contributor
timestamp: May 01, 2014, 11:32:07 am
content: Quote from: akis on May 01, 2014, 07:46:20 amI have thought this previously, but it did not take me long at all to realize this was a poor idea.Using multiple isolated charges will work fine in theory, but in practice you have practical issues like the following;Price - multiple isolated high power supplys cost money, so do multiple charge controllers, multiple larger wires and multiple high current connectorsSize - the above things all also take up spaceEfficiency - all those individual chargers and supplies are much much less efficient than the current designsSpeed of charge - Using a bulk charge, with balancing is much faster because you can use your balance leads as a kelvin connection to measure the actual cell voltage, rather than the voltage at the output of the charger, this allows you to charge much faster unless you use VERY thick wires for balance connection.Complexity - with a more complex system, there is more to go wrong.Many other things too which do not immediately come to mind.Quote from: Psi on April 30, 2014, 08:40:40 amThat is actually quite uncharacteristic of their products; when brand new, their capacity at 1C is within the realm of error of measurement compared to rated spec, for example;