Hot answers tagged

660

Every cell phone (as well as laptop and nearly everything with a rechargeable battery) uses LiIon/LiPo (essentially equivalent for the purposes of this discussion). And you're right: In terms of actual incidences, lithium-ion and lithium-polymer are the safest battery chemistry to be in wide use, bar none. And the only reason this now ubiquitous chemistry ...


81

To use Lipo batteries safely, you must treat them with the same respect you would anything that can store and rapidly release a large amount of chemical and/or electrical energy. The bigger the battery and the lower the internal resistance (e.g. higher C rating) the more you need to be careful. They can be used safely... just like gasoline can be used safely ...


55

When a big company wants to make a LiPo charger, they can: A. Have experts on staff and do extensive testing to make sure the charger will work safely across the full range of operating conditions. B. Buy pre-made ICs or assemblies that have been given that same level of care. C. Subcontract out the work to people who know what they're doing. When you ...


41

[Though this late answer may get little exposure now that the question has exited the hot-list, I think it is essential to further emphasize the contrast between the comprehensive safety features in devices such as laptops and cellphones vs. the typically far less comprehensive safety features in hobbyist or DIY devices.] Context is essential when ...


18

The voltage divider needs to join the MCU in deep-sleep mode then... This can be achieved with a P channel FET (for instance).... When the MCU wakes up, it will want to measure the battery voltage so what it can do is turn on a circuit formed around a P channel FET that connects the battery +V to the voltage divider: - The ADC input is shown to the right ...


14

I recall NASA discovered that Li-ions above 4.10V/cell tend to decompose due to electrolyte oxidation on the cathode (+), while those charged to lower voltages lose capacity due to the solid electrolyte interface (SEI) building up on the anode (-) with Lithium Oxide and Lithium Carbonate. This was verified by Dalhousie University by Dr. Dahn who proved ...


14

The charge (termination) voltage level of Li-ion batteries is specified by manufacturer. The specification is based on reasonably accepted number of cycles ("battery service life") a battery can withstand, say, 500, or 1000. This parameter depends on particular cell chemistry, internal construction, charging current, and is picked up by manufacturer for the ...


13

For this answer I'm going to define LiPo as flat packs, and Li-Ions as 18650s. The reality is more messy than that, but I believe this way of defining the terms will help answer your question as you seem to have intended it. Traditionally, 18650s didn't support high current applications, a continuous current rating of 5A was considered to be high. 18650s ...


13

It is correct. You normally see PMOS connected like this to act as a reverse-polarity "diode". simulate this circuit – Schematic created using CircuitLab It has much less voltage drop than an actual diode and will protect currents from flowing the wrong way when the voltage is connected between the battery terminals are connected in reverse. ...


12

Charging Li-Poly and Li-Ion is not as simple as just pumping a voltage across it. It's a two-stage operation involving both constant current and constant voltage. Apply a constant current of (say) 1C (so for a 1200mAh battery that's 1.2A) until the voltage rises to 4.35V Switch to constant voltage applying 4.35V until the current flattens out (usually ...


8

For a hobbyist project, I'd go with a NiMH option. They are significantly easier to charge (you can charge them with a commodity linear regulator (LM317) and a few passive components), and safer. The weight difference will likely not matter much- your robot doesn't seem to need to change directions very quickly, and it's not something like a quadcopter where ...


8

Those batteries do have built-in protections, but not the kind of ones you see on sparkfun batteries. They have thermal protection required for charging the battery and sometimes low charge cutout device which prevents discharging too much the battery. However, there is no overcurrent protection, overvoltage and noob-proof abuse protections... You have ...


8

Lipos have higher voltage per cell (3.7 vs 3.2). All else being equal this equates to more power. Lipos may also have lower internal resistance and higher maximum discharge rates, which equates to even more power. If you want to make a fair comparison then weight and size must also be taken into account. Lipos are generally lighter and more compact than ...


8

Protection circuits are usually distinct from charging circuits. Many battery packs are designed with the intention of being charged by a dedicated unit that will control the charging process. The charging process may involve cell balancing, if the pack contains a large number of cells in series, generally 4+ cells in series (4S, 14.4V) nominal will ...


8

Because while total power stays the same, voltage droop across a cable due to high currents can be avoided with higher voltages and lower current. Copper is expensive, and it's simpler to deal with 2.5 Amps at 20V than it is 10 Amps at 5V.


7

You can always charge a Li-ion or Li-Po battery at less than its specified rate provided that all the normal charging requirements (CC followed by CV, don't overcharge, don't trickle-charge, don't let the battery get too warm or too cold, etc.) are adhered to.


7

In your post you said that you wanted 1A out at 12V, and that you calculated that you needed 3.24A in with no losses. You didn't state what input voltage you were using, but it can be calculated. 12V * 1 A / 3.24A = 3.7V. 3.7V is much lower than the minimum input voltage of 5.0V specified in the datasheet. So your supply probably won't work right. With ...


7

Ohm's Power Law: $$ P = V*I = I^2 * R $$ Power lost in copper, and needing to be dissipated as heat, goes up with the square of the current. Consumers don't like thick cables or hot laps. So if you can deliver Power to the laptop, and use it within the laptop, with the least amount of current, but still within ELV (Extra-Low Voltage) levels (< 50 ...


7

With the drain and the source connected as you show, it will do what you want — the body diode is pointed in the correct direction, as shown below. But you do need a diode in the USB path, so that the gate can be pulled to ground when the USB is disconnected. simulate this circuit – Schematic created using CircuitLab


6

Charging the battery safely is a primary concern. I typically like to DIY; however when safety is a factor, it would probably be better to buy a product from a quality company. So to give you some ideas, this is a "smart" charger you can set for various current and voltage settings (up to 9.6 volts); it will also turn off automatically. It will also charge ...


6

Pretty much most boost converters do what you want. Anything that uses 1 or 2 AA for a usb charger have this. The MintyBoost is a popular (and fully detailed) diy kit for boosting 2 NiMH or Alkaline to 5v at 500mA. It is based on the dip LT1302 with the usual passive components needed.


6

I would try to get one of these unsold phones and the wall power supply. Than use the phone as charger. So you eliminate any mechanical problems, you have near to perfect contact and you can be sure, that no lipo is burned. As a bonus you get a quite reasonable Display, if the lipo is still charging. I use an old Nokia 3310 for that matter for two years now ...


6

All great answers. Here is a short one. A 7.4 volt. 5 amp-hour battery has 37 watt-hours of energy, or 133,200 joules. Compare to a .357 Magnum's 873 joules of muzzle energy. The trick is not to let it a bunch of it out at once by overheating or crushing.


6

You actually have a few issues when driving a DC motor with more than the boilerplate voltage rating. Max Current: At no time should you exceed the maximum current rating of the motor. It will overheat and can in fact demagnetize itself. As such you need a current regulation or limiting circuit to keep the applied current sufficiently under the max. Over-...


6

If a lithium-ion battery with more than one cell has no provision for balancing, it's a blatant safety risk. As a battery ages, one cell will drop in capacity faster than the other, causing the lower capacity cell to eventually overcharge (risk of fire!) during charging. Nobody but the very cheapest, least reputable sellers will sell you one. That said, ...


6

Not sure which "thingy" you refer to. The smaller lump has protection circuits. But the secret sauce to these emergency car starter power packs is in the big lump. The reason they can put out so much current (hundreds of Amps) despite being such small capacity (e.g. 8 Ah) is because they use lithium iron phosphate cells. Lithium iron phosphate batteries ...


5

I've built several products around different chemistries. I have found LiPo the easiest to use since there are already specialised charger IC's that do all the work however, like you said, having them in series is not that straight forward. I believe the NiMH is a better solution if you don't have a lot of experience with chargers or if you don't find a good ...


5

You should really get rid of the whole pack and don't absolutely try to charge it. These kind of batteries have very large flat metal pads soldered to a big pcb and it seems very unlikely to me that something fall off: your cells are probably both dead. You might carefully try to rip off the heat-shrink enclosing and carefully desolder the maybe-but-not-so-...


5

Amongst other things I design solar charged lights. I want customers to be able to put a "dead" light in a dark place for a long while without destroying the battery. My approach is to reduce off current to so close to zero as to not matter and then deal with battery self discharge issues. 1 uA = 8.8 mAh/year. Scale that for time and discharge rate as ...


5

Something like this could be very efficient (not much more than the LED draw) with a CMOS op-amp. For fairly constant visual brightness (+/-10% current) you could derive the 100mV with a voltage divider from the battery, or use a low current reference such as the TLV431 which would require another 100uA or so but would make the current very constant and ...


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