Charging Batteries - Page 1
Forum section: electronics / beginners

author: custardinc
author_rank: Newbie
timestamp: November 22, 2014, 02:51:43 pm
content: I want to first say that I am very inexperienced with electronics, I know the basics but that's about it.I'm wondering if there's any way to charge a laptop/tablet battery with an external USB charger? Specifically an EasyAcc PB18000. It's got a 2.1A and a 1A USB socket, I believe they can both be used at the same time for a total of 3A. The laptop charger runs at 19.5V, 2A. Apparently it's possible to step 5V up to 20V, with a loss in current to 1/4 (I'm aware of V=IR / Ohms law). I've got a couple of questions about this though.When charging a tablet battery (Sony Vaio Tap 11) is it more important that the voltage matches that of the charger, or that the current does?Will it damage the battery if either of them don't match?How exactly would I go about setting this up, would I need to modify the external battery?Thanks in advance, I hope this is the right place for advice!

author: Tandy
author_rank: Frequent Contributor
timestamp: November 22, 2014, 03:16:21 pm
content: The input voltage to charge the device (tablet) must be correct, to low and it will not charge. If the tablet has a suitable regulation circuit in it a little over voltage may be ok, but unless you know for sure it is not worth risking.A lower current will just mean that the tablet takes longer to charge, or if used while the charging battery is connected may not charge at all. The problem however is that you will be drawing maximum current from the battery pack during charging and that will quickly drain the battery and reduce its life.

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 05:19:59 pm
content: Laptop/tablet batteries are charged inside the computer when power is applied to the computer. You don't make it clear what you are trying to do here,but you should not try to charge the battery when it is not in the computer.You can substitute the original computer power supply with a different one that matches its specification (19.5 V, 2 A). The match is not critical: the voltage could go a bit lower, and the available current could be a bit less. If the computer or tablet determines there is less power available it may go into a "low power" mode, and it may turn off the battery charger and stop charging the battery. (This is what happens with the power outlets on airplanes: my computer says "not charging" when I plug it in on a plane.)So if you want to replace the original charger, it is best to aim for about 19 V and about 2 A if you can.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 22, 2014, 05:36:09 pm
content: Are you asking about charging the battery INSIDE the tablet, or have you removed the battery and are asking about charging it externally?If you are charging the battery INSIDE the tablet, then there is no substitute for using the 19.5VNo, you cannot use any kind of 5V USB charger, no matter how much current it supplies.You must have the proper power connector and the designed voltage (19.5V) to make the tablet's internal charger operate properly.If you have somehow removed the battery from the tablet, then you must have a proper charging and monitoring circuit or you have an excellent chance of damaging the battery. Many modern types of batteries can fail with significant explosion and damage if not charged properly. That is why there are restrictions on loose batteries traveling on commercial airlines, for example.Unless you are trapped on a desert island somewhere, converting 5V to 19.5V is far more trouble than it is worth. Use a proper 19.5V source. You can make one from scratch easier than converting 5V.

author: custardinc
author_rank: Newbie
timestamp: November 22, 2014, 07:18:24 pm
content: Perhaps I should've just posed the question How to step 5V 2.1A up to 19.5V 2A. Although apparently it's not worth it.The reason I asked is because the battery is not removable, so I can't just get a spare, and I have several USB battery banks. The tablet has a capacity of 3800mAh and the external bank is 18,000mAh, so utilising this would give me several hours extra hours of usage.

author: Richard Crowley
author_rank: Super Contributor
timestamp: November 22, 2014, 07:31:53 pm
content: Portable computers use custom-designed battery-packs (or hard-wired internal batteries).Most modern battery chemistries require special charging procedures, including monitoring battery temperature to avoid explosion.Most portable computers require somewhere around 19~20V to allow for losses in the internal charging circuits.There are portable battery packs that provide the 19~20V required by portable computers. Specifically for your scenario.5V @ 2.1A = 10.5 watts. 19.5V @ 2A = 39 watts. You need 4x more power than you have available.And that doesn't even consider that conversion is nowhere near 100% efficient.

author: Tandy
author_rank: Frequent Contributor
timestamp: November 22, 2014, 07:36:59 pm
content: You could conceivably connect 4 of the 5V batteries in series to get 20V but they would all need to be the same. Probably better off finding a battery pack of the correct voltage to use as your charging source.

author: VK5RC
author_rank: Unknown
timestamp: November 22, 2014, 10:57:59 pm
content: Remember the Ah capacity is at the rated voltage, so your 5v bank when boosted to 20v (or so) will only have 1/4 of the "capacity" probably closer 1/6 with conversion losses.( The laptop will think it's on mains so will start charging the battery as well, so time will be reduced further! Unfortunately this is some of the problems of intelligent chargers needed for today's compact batteries.) Edit see Ian Bs comments above and below.

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2014, 11:06:39 pm
content: Quote from: VK5RC on November 22, 2014, 10:57:59 pmMost laptops will stop charging the battery when input power is limited. See my airplane comment above.Also (for instance), my iPad will not charge the battery when plugged into a normal USB port, even though my iPhone (smaller battery) will.

author: eas
author_rank: Frequent Contributor
timestamp: November 28, 2014, 11:57:27 pm
content: Quote from: VK5RC on November 22, 2014, 10:57:59 pmActually, for USB power banks, the Ah capacity is usually at the cell voltage.Quote from: IanB on November 22, 2014, 11:06:39 pmSome laptops have a signaling mechanism with the external charger, that they use to determine whether the supply is power limited. Of those, some default to a lower power mode if the signal isn't available.iOS devices use a similar scheme. They'll default to something like 500mA if they can't determine that the USB power source can supply more. That is actually enough to charge an iPad, slowly, but you'd never be able to tell by watching it, because when the backlight is lit, there isn't enough power to charge too. It only charges when the display is off.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: November 29, 2014, 01:03:34 am
content: Quote from: IanB on November 22, 2014, 11:06:39 pmIt says it won't, but it will. Plug it in, shut off the display, leave it for an hour, wake it back up, and you'll see that the battery has been charged. It'll charge just fine on a 500mA source as long as you're not using it at the same time.

author: G7PSK
author_rank: Super Contributor
timestamp: November 29, 2014, 09:33:38 am
content: You could try a DC to DC step up converter. I have used them to run laptops in vehicles, they can be had very cheaply from e bay. This one may not be the best for your use but it will give you an idea of what to look for.http://www.ebay.co.uk/itm/New-150W-DC-DC-Boost-Converter-10-32V-to-12-35V-6A-Step-Up-Voltage-Charger-Power-/281341818589?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item41814776ddOne of these would be better for your use.http://www.ebay.co.uk/itm/5-30V-to-0-8-28V-DC-Boost-Buck-Converter-5A-Constant-Current-Volt-Regulator-/251406635784?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3a8900d308

author: Paul Price
author_rank: Super Contributor
timestamp: November 29, 2014, 10:13:22 am
content: There is a chance what you are asking is how to charge a high capacity battery at 19 to 20 V that can be used to replace the wallwart for running your laptop or charging its battery when no AC power is available.In that case stepping up 5V to 19 to 20V to charge the high capacity battery would work fine(even if slow and at a low current) if you make a flyback converter to use your USB 5V to make 20V. The step up converter must be regulated so the output does not exceed the maximum charge voltage of the high capacity battery pack but a low charge current would not be a problem.