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There is this Bluetooth Audio Reciever/Amplifier board that I'm looking to get from ebay, and according to the diagram, there's a MicroUSB 3.7V lithium battery charger component in it.

With my understanding that wiring multiple batteries don't increase voltge but rather current/capacity, will I be able to wire in multiple 3.7V 18650 batteries in parallel (maybe 4) in order to increase my runtime, at the cost of having longer charging times?

Is this a universal case or a chip specific situation? I've asked all of the sellers who sell that product, and none of them knows the answer.

Link to eBay item: http://www.ebay.ca/itm/181744265268

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There are a number of possible issues. Too many to list in a comment. So I will list them here instead.

First, safety. If each cell has protection already, then that is good. I would definitely recommend you use protected cells in this application. But if not, at least make sure you insert some kind of current limiting device between charger and each cell. This could be a fuse or a PTC (self-resetting fuse). The big issue with cells in parallel is that if one starts to fail, the other cells will dump all their charge into the failing cell, making matters worse. So some type of current limiter is needed to cover this case. A fuse is probably safer, but a PTC would also be reasonable, and can recover automatically if it ever trips during normal operation.

pack topology

Another issue is that when you initially connect the cells, you should make sure that they are already at the same voltage, or if not, then limit the current until they equalize. The equalization current could be very large for 18650's otherwise (and if you use fuses, the equalization current could cause them to blow). The easiest way to deal with this might just be to individually charge each cell fully before connecting them. The voltages will be very close at that point.

When cells are in parallel, there is probably not a big concern with slight capacity mismatch if they are the same cell make and model. If they are not matched perfectly, they will just naturally share current proportionately to their capacity during charge and discharge, which is what we want anyway.

Now let's consider the operational problems. Chargers are normally set up to charge a cell of a specific capacity. In this case, we don't know what that capacity is, but it is probably not 4 x 18650. Small variations in capacity won't matter, but if you use 4x the design capacity of the charger, there could be some problems. First off, the charge rate will be slow for such a large battery bank. Second, the charge might not terminate properly. The normal charge termination criterion is based on the acceptance of the cell during the constant voltage stage of charging. With 4x capacity, the acceptance may never decay to the low limit required for normal charge termination. Also, chargers normally have a secondary, backup termination criterion. Basically, if the normal charge criterion is not met within a timeout period, charge will terminate anyway. In this case, the timeout may even occur before the batteries are fully charged. Normally, the timeout would only occur if the cell is faulty, but since the capacity is much larger than anticipated, it could trigger prematurely.

Hope this helps! Note that by listing problems, I am not trying to discourage you from trying this. I just want you to be aware of the potential problems so that you will recognize them if they occur.

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  • \$\begingroup\$ I've gotten the cell-match thing figured out and plan on doing it precicely, and I don't mind having a really slow charge time, I'm already used to waiting 16 hours to charge a 20Ah usb charger. Safety-wise, would a MF-RHT200 polyfuse be a good choice for me, with its 3.8A trip current and 2A hold current? Would I need just one or four of them (one for each battery)? Also, what are you suggesting by a backup termination? \$\endgroup\$ – user3125996 Nov 3 '15 at 5:19
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    \$\begingroup\$ What I am saying is that if the charger IC is expecting 1 cell, and you attach 4 cells, it may timeout before the cells reach full charge. Most charger IC's have such a timeout. It is very hard to recommend a PTC without knowing the normal current consumption, etc. But on that datasheet, the MF-RHT200 is probably the best choice. It seems like the MF-RHT series is designed for high temperature applications. I would probably try to select a more standard one. It is kind of a good feature that the PTC trips at lower currents when it is hot. This makes it more sensitive if the cell is hot. \$\endgroup\$ – mkeith Nov 3 '15 at 5:35
  • \$\begingroup\$ Alright, so I'm guessing the 1 polyfuse should go in between the power input and after all the paralleled batteries right? Personally, I think that polyfuse I picked out looks like the right one but if you have another model to suggest to me, go ahead. Another version of the seemingly same bluetooth board seems to suggest a greater than 1.5A battery amp handling, so I'd imagine the max would be in that area but not much higher. Would the main inconvenience of a timing out charger be I may need to un/replug the usb cable a few times to get a full charge? \$\endgroup\$ – user3125996 Nov 3 '15 at 6:20
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    \$\begingroup\$ You need one polyfuse for each cell (but only if the cells are not protected). \$\endgroup\$ – mkeith Nov 3 '15 at 6:22
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    \$\begingroup\$ I think that if the cell is protected, there is no need for PTC's. Check the candlepower forums for recommendations. The two I know are EagleTac and Orbtronic. They both use panasonic cells. There are a lot of extreme low quality (basically fake) 18650's available, so be careful. \$\endgroup\$ – mkeith Nov 3 '15 at 23:42
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Yes, but you had better make damn sure the cells are well matched since more than a very small difference can cause issues. And charge them all to the same voltage before putting them together. You probably even want to connect all the negative terminals together and put largish (1kohm+) resistors between the positive terminals and leave them overnight to allow all the cells time to balance before connecting them together directly.

Edit:

Balancing circuit:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I had a feeling cell-capacity matching was going to be a factor. What about if I were to buy a new 4-pack of 18650s (probably VTC5s), and immediately hook them up directly to the bluetooth/charging board in parallel without ever touching them anywhere else charger-wise, would that be enough? \$\endgroup\$ – user3125996 Nov 3 '15 at 4:14
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    \$\begingroup\$ I would still allow them to balance overnight. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 3 '15 at 4:22
  • \$\begingroup\$ Alright, so I should charge the 4 batteries up until it stops one-by-one using that bluetooth board's charger, then outside of the charger I'd connect the negative leads of each battery together, and the negative leads together with a 1kohm+ resistor? \$\endgroup\$ – user3125996 Nov 3 '15 at 4:29
  • \$\begingroup\$ Can I have a simple visual diagram of this setup so I have a clear picture of what you mean? \$\endgroup\$ – user3125996 Nov 3 '15 at 4:30
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    \$\begingroup\$ They're all connected to the same return node; that's what that downward-pointing triangle is. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 3 '15 at 4:54
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Contrary to what others may promote as OK I cannot ever recommend that you place batteries in direct parallel connection. That goes for charging and for discharging into a load.

For lithium type batteries it is wise advice to charge them individually.

If you must have a larger capacity during the usage discharge into a load it is best to connect all the cells in series and then use a switching regulator to convert the total stack voltage to that which is required for your load.

This caution is especially important for persons that only have the barest of understanding as to why my recommendation exists as it does.

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    \$\begingroup\$ Any rationale for this? In my opinion, series connection is more dangerous as it requires a balancer for safe charging. Also the per-cell protection chips are largely useless for 3 or more cells in series, because they will break down at ~8V reverse voltage. \$\endgroup\$ – jpa Nov 3 '15 at 6:17
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    \$\begingroup\$ @jpa - I said to charge the cells individually. \$\endgroup\$ – Michael Karas Nov 3 '15 at 9:24
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    \$\begingroup\$ The problems with protection chips also apply during usage and especially for end-of-discharge. Safely operating a li-ion series pack requires a separate end-of-discharge switch, the per-cell protection chips won't be enough. Would you explain what risks you see in parallel usage? "Especially important for persons who do not know why it is important" does not really help much. \$\endgroup\$ – jpa Nov 3 '15 at 10:05
  • \$\begingroup\$ An open or shorted cell in a series pack causes the pack to produce insufficient voltage & power--which renders the user device useless. This is a good indicator to the user that the battery or the device requires service. No other cells in the pack are usually damaged. An open or shorted cell in a parallel pack causes the capacity & run-time of the pack to be reduced. The user device will often continue to operate normally for a while; however, a shorted or partially shorted cell will eventually... \$\endgroup\$ – zeffur Nov 3 '15 at 18:37
  • \$\begingroup\$ drain nearby cells past their recommended cut-off voltage--which will eventually result in irrecoverable damage to the other cells in the pack, if the bad cell is not replaced. But, none of that means people can't or shouldn't build & use a parallel or series+parallel battery pack. In fact, series+parallel packs are very often made in order to get the ideal V & capacity from a battery (all EV cars use series+parallel batteries). \$\endgroup\$ – zeffur Nov 3 '15 at 18:37
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"Yes", if you have the right charger--"No" if you don't have the right charger.

You seem to have an error in your understanding of the product that you linked above. The 2-pin port that is shown at the url is for connectivity to a DC power supply (i.e. 3.7V cell or battery or a 5V USB port)--it does NOT appear to be a charging port. To the right of that port is a manual switch to switch the supply source from 3.7V to 5V.

Now, as for charging li-ion cells in parallel, your cells should have the same charging & discharging profiles/characteristics to be safely placed in a parallel battery (if they don't match each other well then they will not discharge & recharge at the same rates, potentially drain other cells, potentially over-heat during dis/charging, & possibly vent/explode &/ ignite while charging). The cells should all be initially charged to the same level & allowed to balance/equalize in a parallel connectivity arrangement for 24 hours. I would also recommend current limiting resistors in case a cell/s short. That should limit the risk of fire.

Even if you initially start off with ideally matched cells that function well for a while, it is likely that the cells will become unbalanced as they age. The only way to mitigate that problem somewhat is to occasionally manually charge them, balance/equalize them, & replace them when the differences between the cells become too great.

You can also undercharge the cells (e.g. charge them to 4.0 or 4.1V instead of 4.2V) to reduce overheating that will occur during charging as the cells age & become imbalanced.

A parallel battery is doable with li-ion cells, but it is a bit risky & a bit of a hassle as the cells age.

best regards!

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    \$\begingroup\$ I've questioned that a few times too, however there are multiple pieces of information that hint at a presence of an internal charger. Examples include "Best 3.7 V recharg lithium battery", "Rechargable lamp (red)" (status led), "Micro USB plug 5V power/charging", and "(lithium battery or power need >1.5A )". If this turns out to be true that there's no internal charger, I plan to add a switch to switch between being connected to the board and a standalone microusb charger chip, no biggie. \$\endgroup\$ – user3125996 Nov 3 '15 at 14:33
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    \$\begingroup\$ All poorly written English on their part. If you have a charge port you must also have a power supply port to provide charge energy, right? Where is the power supply connector on the board? There is no such indication on the annotated photos. \$\endgroup\$ – zeffur Nov 3 '15 at 17:50
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    \$\begingroup\$ The MicroUSB plug? I've seen a ton of chinese MicroUSB powered 3.7V lithium-ion chargers go for very cheap on ebay, so if this board does do charging, I wouldn't be surprised if one of those may have been integrated into it. Again I have no problem using a switch to go between the charger being connected to the board and the standalone usb charger if I end up having to do that. \$\endgroup\$ – user3125996 Nov 3 '15 at 18:37
  • \$\begingroup\$ You may be correct, but I think "DC 3.7V-5V (Best 3.7V recharg battery)" is them recommending you use a rechargeable li-ion cell when you power the device through that connector. The "DC power supply USB/Battery choose" switch is an odd item. That may be there to prevent over voltage--in case both power in ports are connected simultaneously. \$\endgroup\$ – zeffur Nov 3 '15 at 19:14

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