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Forum section: electronics / beginners

author: The_Almighty_Bacon_Lord
author_rank: Contributor
timestamp: April 24, 2016, 08:17:40 pm
content: Delete Delete Delete

author: ConKbot
author_rank: Super Contributor
timestamp: April 24, 2016, 09:53:18 pm
content: The first board (green ones) looks like they doubled up on the mosfets and have better heat sinking, and it looks like they both use AO4407A mosfets, (kind of hard to read on the photos of the green PCBs) So that gives a slight theoretical advantage to it? But nothing really compelling either way.The battery monitor IC used on the blue PCB (http://www.sinowealth.com/ftp//BMS/BMS_SH367XXX/SH367003/SH367003CV1.0.pdf) looks pretty standard, but given that I cant read Chinese, and cant read the part number on the main IC on top the green PCBs I cant really compare them.As far as charging, most battery protection circuits like this will disable the corresponding mosfet (charge or discharge) so that the opposite action can still be performed. I.e. undercharged battery, disable the discharge mosfet, and the pack can still be charged though the body diode of the discharge mosfet which is turned off.Are you buying pre-tabbed cells (little nickel strips spot welded to the battery), using a holder, or trying to directly solder the cells? The first 2 are perfectly safe, just be mindful of working with live batteries. Soldering directly to the cells is definitely not recommended by the cell manufacturers, and not acceptable for any professionally made things. But for DIY, if you can do it quickly enough to not overheat the battery, it can be done with minimal damage to the cell.You want a large soldering iron tip with the heat turned up (700-750F) Roughen the surface where you want to solder lightly, clean it, apply flux, pre-tin your wire. When you go to solder your wire down, hold the wire against the battery, iron on top of the wire, and add your solder to between the wire and the battery. You should be able to quickly see the solder start to wet the surface of the battery. Take the iron off, hold the wire in place still, blow on it to cool it quickly (not too hard, dont blow the solder off the joint ;p ) You should be able to do the joint in under 3 seconds from when the solder in the wire melts to when the the surface of the battery is wetted. If not, you need a bigger soldering iron tip.If you take too long, you can overheat the internal components in the battery, cause an internal short, followed shortly by the battery venting. This is no joke, as Ive seen tales on candlepowerforums of people who have scarred lungs, asthma, etc caused by breathing fumes from vented cells. (usually from imbalanced cells, I dont recall any from during soldering. Perhaps any Chinese literate members on here can say if the IC linked above supports balancing? ) I'm not one to fear-monger about things on here either, and Li-Ion cells still earn a good bit of caution from me. ( Also, take any advice you see on RC hobby websites when it comes to electronics with a grain of salt or 10 )Work on a fireproof surface, or have a sand bucket near by so if you do have a cell venting, you can focus on evacuating the area, then ventilating, rather than preventing the damn thing from burning down your workshop before you evacuate. Be conscious of the risks and address them, dont be paranoid.http://www.batteryspace.com/will sell pre-tabbed cells if you want to avoid soldering onto cells directly, however I'm not sure if they ship to Canada. They also sell protection and protection/balance PCBs too.

author: ConKbot
author_rank: Super Contributor
timestamp: April 25, 2016, 02:09:59 am
content: Quote from: The_Almighty_Bacon_Lord on April 24, 2016, 11:40:52 pmNope, they are normally sealed shut. When the cell vent, a thin stamped section of metal ruptures and lets the excess pressure out. After that the electrolyte and flammable solvent used can evaporate out. Plus the cell venting is supposed to disconnect the internal cell from the positive terminal of the casing (at least in some battery designs, I'm not sure if some omit this) Either way the cells will have lost some capacity, and will be unsafe to use, and definitely unsafe to put in a series pack.Quote from: The_Almighty_Bacon_Lord on April 24, 2016, 11:40:52 pmThe MOSFETs are the switching elements. Because of the body diode (intrinsic to MOSFET construction, you cant get rid of it) the MOSFET can only block current in one direction, so the 2 are facing opposite directions. 2 of them is the minimum amount needed. The board with 4 has 2 in parallel to control charge current, 2 in parallel for discharge current. Paralleling the MOSFETs will reduce the voltage dropped across them, and improve how much current it can handle, and reduce the power dissipated. However all that is assuming that is the MOSFETs arent counterfeit, ant the ones pictures are the ones on the board. Unless you plan at running it near the 4-5A limit constantly, then its not really a concern.Quote from: The_Almighty_Bacon_Lord on April 24, 2016, 11:40:52 pmI dont see anything in the auction saying that it would balance the pack, so I wouldn't assume that it would. As far as the voltage matching, when you initially assemble a pack, all of the cells should be charged fully (or discharged to the same state, but its much easier to just charge them fully) If the cells dont have an equal capacity and internal resistance, then as you cycle the pack, charging and discharging they can become imbalanced. I.e. you have 2 2500mAh cells, and 1 that is 2400mAh (less than the nominal 5%-10% tolerance you can expect) the 2400mAh cell will discharge first. The protection circuit should prevent it from being overdischarged though. When battery pack manufacturers make high quality packs, they can buy thousands of cells and match ones that have a very close capacity so the packs can stay balanced. For example, a pack I assembled with cells I bought loose, when discharged the cell voltages would vary by a few 10s of milivolts. A pack I assembled with cells harvested from a new battery pack for a laptop (good sale from toshiba, bought them specifically to tear apart) they all were within 1mV after discharging.This limits the available capacity in the pack, and over time, if the imbalance gets worse the protection circuit will be kicking in more often greatly reducing pack capacity. Laptop packs used to (not sure if its currently the same) not include balancing circuitry, relying solely on the original balancing and matching. Hence why you can get laptop batteries with only a few bad cells, the rest still functional. When weak cells overcharge or overdischarge, the protection circuit cuts off the whole pack.Quote from: The_Almighty_Bacon_Lord on April 24, 2016, 11:40:52 pmOnce your pack is assembled, all your charging and discharging will be done through the P+ and P- connections on the battery protection circuits. For 3S, you'd be charging with a 12.6V source, current limited to 1C (1* your battery capacity, 2.5A for a 2500mAh pack, 3.2A for a 3200mAh pack, etc), and stop charging once the current falls to C/10-C/30 (250mA-83 mA for a 2500mAh pack)Seehttp://batteryuniversity.com/learn/article/charging_lithium_ion_batteriesfor more information about charging, and definitely use them for learning more about batteries.Once you do build the pack, for the first few cycles, checking the voltage with voltmeter before and after charging may be prudent until you know its performing correctly.