An old tablet I was playing with, "bricked" and won't charge, so I'm looking to jump charge the battery directly.
It's a 4000mAh LiPo 1S battery, so a 5V charging voltage should be enough. I was wondering if I could use a conventional 5V wall charger, or do I need a specific battery charger like this?
No, a 5V source is not suitable for this.
Battery management systems will refuse to charge cells that are under-voltage because it generally isn't safe to do that. You can, however, manually jump-charge them.
First measure the voltage across the cell, to verify that it is low. Then use a current-limited bench power supply, set the current limit to around 250mA and the voltage to whatever the cell is currently at plus 200mV. Connect it, and watch the voltage slowly come up. As the cell voltage climbs, the current draw from the supply will drop. Bump the voltage by another 100mV and keep going. It takes some patience, but you'll eventually get the cell to creep back up to a minimum level of charge and the BMS should then accept it.
If you try to apply a large change in voltage to the cell without a current limiter, a huge current will flow through the battery and either blow the internal fuse (if you're lucky and it has one) or set it on fire.
While you could just dial in 3V and a 250mA limit on a proper bench supply, doing the whole jump-start in one run, it's safer to manually set the voltage in small increasing steps in case your bench supply's current limiting circuit has a delayed reaction during load transients.
It goes without saying that lithium battery fires are serious. If you slip you could easily short the terminals and start a fire. The jump starting process could also start a fire. Make sure you've got a powder type fire extinguisher to hand, and ideally a sand bucket.
The general rules for charging a high-capacity Lipo cell are:-
Limit current to a maximum of 0.5C, ie. 4Ah * 0.5 = 2A.
If the cell voltage is below 3.0 V then limit charging current to the normal rate divided by 10, ie. 0.2 A.
If the cell voltage is below 2.5 V then it's permanently damaged and you should not try to recharge it.
Don't let the voltage go above 4.2 V. At 0.2 A this will take many hours, so just regularly monitor the voltage and disconnect the charging source when it gets above 3.7 V.
Do not try to charge a battery that is puffed up, physically damaged, leaking or has a strong smell.
NEVER manually charge a Lipo battery unattended!
For added safety you should keep the battery away from combustible materials, have a smoke alarm on the ceiling (or charge outside), and keep a bucket of water nearby to throw the battery into if it shows any sign of 'cooking off' (getting warm, puffing up, smoke, flames!).
To limit charging current you just need a resistor in series. Assuming the cell is at the minimum acceptable 2.5 V and the power supply puts out 5 V, the resistance required is ~(5-2.5)V / 0.2 A = 12.5 Ω. The resistor should be rated for well over (5-2.5)V * 0.2 A = 0.5 W (ie. 1 W or higher). A higher resistance value is fine, it will just take longer.
As cell voltage rises during charging the voltage difference gets less so the charging current will reduce. This provides a bit of extra safety. If the voltage stops rising altogether or starts dropping before reaching 3.7 V the cell is probably faulty.
Of course not, because your wall adapter is a 5V constant voltage power supply so it is not a charger.
The actual charging circuitry needed is one the tablet circuit board between power supply and battery.
You need a charger, but the one you linked to is likely more powerful and advanced and expensive than what you need. Simpler and cheaper options exist to charge Lithium cells with 5V via USB for example.
