Charging circuit or battery fault?

[sorcerer]

Got an HP Stream netbook in with a suspect battery. Client wants to know how much a new battery will be before deciding whether to go ahead or not.

The battery in there at the moment is completely flat and has no charge whatsoever. Testing on the motherboard side of point A shows 19V into the board, but testing at point B, the motherboard side of the battery connector, the highest voltage I can get is 3.2V, with or without the battery being connected.

The battery is labelled as being 11.4V so I would expect charging voltage to be somewhat over 12V. Does anyone know if the charging circuit is permanently active (in which case I should be seeing much more than 3.2V), or does it only fire up and become active once it sees a good battery that it can charge?

I don't want her spending £30-£40 or even more on a new battery, only to find that it quickly runs down because the charging circuit is knackered and it's not the existing battery at fault after all.

EDIT - I know that the battery connector is not fully pushed home in the photo, but that's not the problem. Believe me, by the time I took the photo the connector had been in, out, in, out and shaken all about

 

[Porthos]

HP Stream netbook

I hate to say this but all of those are disposable and should treated as such. Not really cost effective to even open them. I look at those as if it is not a software issue that a nuke&pave cant fix toss it.

 

[Markverhyden]

A common practice in building electronic circuits is to employ a "trickle" voltage which is monitored at many points to determine health. An example is with alarm sensors. Say the normal sensor is closed. So you see 12vdc closed and when it's open you see something other than 0, maybe 2-3 volts, depending on the resistor used to bridge.

Computer power supplies do something similar.

 

[NviGate Systems]

Lithiums charge between 3-4 volts for most designs. So that charge side voltage actually seems normal.

It's not uncommon for these to die in poorly designed machines.

It's like 90% likely to be the battery but if you want to proceed you need to inform your customer that the battery is final sale and there is labor to be paid. Have them sign off on that.

Or tell them to get a new machine. If it's just the battery, the units still boots right? Just you have to leave it plugged in yes?

 

[Diggs]

Me thinks you are over-thinking this. For $20 buy a cheapo battery on Ebay and see if it charges. It should be covered in your diagnostic fee anyway.

..and as @Markverhyden said, lithium ion packs are extensively internally wired and monitored at every cells you can't really probe them with a VOM like you could a NiCad.

 

[gadgetfixup]

Most laptop motherboards run a dual mosfet design with the laptop pulling it's main voltage from the power side of the mosfets (middle). The mosfets act as protection devices to the main voltage rail. So on the charging side the mosfets will typically check there is power on the center pin before letting full amperage from the inner and outer barrel connections. So the middle pin powers the switching voltage of the mosfet. On the battery side the mosfet protects against reverse polarity and overcharging.

So to do some more voltage checking look at the 8 pin chip near your charging plug and battery and check voltages there. On the charging side you should see your 19V on 7 pins (3 inputs on one side and 4 outputs on the opposite side) the trigger voltage on the input side could be either higher or lower than the 19V depending on the type of mosfet it is (8th pin). You can look at the mfg schematic for the chip to better understand why it's higher or lower but the point is the trigger is there and switching the mosfet on or your laptop wouldn't power on from the charger.

Find the mosfet for the battery side of the power rail and check with the charger connected and find you have 19V on 4 pins (they are connected to the charging output side of the first mosfet so if there is power on the output side of that mosfet there is power on the battery mosfet. Once you find this mosfet (8 legs) you can look at the component schematic and determine what voltage triggers this mosfet (oftentimes the charging mosfet and battery mosfet are the same chip number) allowing the battery to receive a charge up to the point it's charged and then it shuts off this battery mosfet. Many times it's lower voltage the battery supplies except in the instance it's been run completely flat. (We run into this many times with Samsung tablets where manually charging them with a power supply and plugging them back into the tablet will flip the battery mosfet back on so the battery will charge).

Have you tried charging the battery with a regulated power supply to get some voltage back in it if it takes a charge at all? A regulated power supply should be part of any repair shop equipment. In this case, we'd hook that dead battery up and immediately know what sort of amperage it is taking and know if it's good or bad. If we put 12V into it and it doesn't take much amperage it's junk. Most of these batteries when good will take the rated voltage and draw 2A or more until charged.

HTH

 

[sorcerer]

Thanks folks, especially 'inbargains' for that very in-depth reply.

New battery obtained and machine now working well.

Cheers all.