Inexpensively Learn about your GPS battery charging


A lot of questions arise here regarding folks worried about their device's battery, various charging methods, etc., etc.

You can learn a lot about your various devices charged via USB by buying a simple device. I'm sure these can be widely found but I bought the two I have on eBay. I can't guarantee that this is the best price or the vendor I used, but it is a good price from a US vendor with great feedback. If patient regarding delivery times, you can find them even cheaper from Chinese vendors. Check it out:

What can you learn by adding one of these between your USB output socket and device being charged? Let me count some ways:

-You will see the instantaneous voltage and current being sent to your GPS and this is a great way to distinguish between low power 0.5A chargers and the 1-2A chargers which can charge your GPS much more quickly.

-As your GPS device's battery approaches a full charge, you will see the current being drawn by the device drop from its max to a smaller value, eventually dropping to 0.000A letting you know the GPS is fully charged and the device's battery protection circuitry is preventing overcharging.

-If charging a fully depleted GPS battery, you can see how many mAh was sent to the GPS and by comparing that value to the capacity of the original battery, you may learn about how well an aging battery is doing compared to a new one. You also see the time it took for a full charge to occur given the charger's power and state of the battery when starting the charge.

-Even more fun is using one of these for your cell phone as it charges, especially when some cell phones and cell phone chargers use a form of Fast Charging. This device will show how the Fast Charging works, whether it's a higher amperage charger, or for devices like many Samsungs, that the charger sees a device capable of Fast Charging and capable of accepting charging voltages of 9 or even 12V. Given the number of counterfeit Samsung Adaptive Fast Chargers being offered on eBay by Asian Sellers at suspiciously low prices, you may well find that the counterfeits do not offer 9V or 12V charging when the charger senses a 9 or 12V capable device to be charged and they simply offer higher current charging at 5V. For this reason, avoid buying a USB power measuring device that can not show much more than 5V. Several cheaper devices won't show USB voltages above about 7.5V making them useless when monitoring a Fast Samsung charger trying to recharge a Fast Charge capable Samsung device.

A couple YouTubes: (Go to about 2min 30 seconds to avoid the sponsorship video)

Great find ...

and excellent write up CraigW! Thanks.

Alan - Android Auto, DriveLuxe 51LMT-S, DriveLuxe 50LMTHD, Nuvi 3597LMTHD, Oregon 550T, Nuvi 855, Nuvi 755T, Lowrance Endura Sierra, Bosch Nyon

Thanks Craig. An interesting

Thanks Craig. An interesting find!

Maps -> Wife -> Garmin 12XL -> StreetPilot 2610 -> Nuvi 660 (blown speaker) -> Nuvi 3790LMT


Good information.

Good Advice

After reading your post I ordered one of these off eBay for $4...sekker located in US...fingers crossed.

I can think of at least a dozen devices to try this on. Thanks !!


Thanks for posting CraigW. I too ordered one. US $ 2.43 Cdn $ 3.70 free shipping from China. If it takes more than three months, I don't care since I won't be home until the end of April.

Nuvi 2797LMT, DriveSmart 50 LMT-HD, Using Windows 10. DashCam A108C with GPS.

Good Job Craig

Thanks for the two links and information you posted.
This can be very useful for some of our members that are interested in their battery's state of charge.


Nuvi 2460LMT 2 Units


Melaqueman wrote:

Thanks for posting CraigW. I too ordered one. US $ 2.43 Cdn $ 3.70 free shipping from China. If it takes more than three months, I don't care since I won't be home until the end of April.

I'm pretty sure the two times I ordered one of these Keweisi devices, I also ordered from China from a high-feedback seller. For the link in this thread, I made sure to find a US Seller with great feedback but at a higher price since I didn't want to be responsible for a bad delivery experience for the users here. Yup, if you're willing to wait 3-7 weeks with maybe a 5% chance of it being lost in the mail/Customs and refunded, you could buy a good 2-3 of these testers from Asia for the price of one from the USA. Of course, for someone living in Canada/Mexico and only traversing the USA a couple times a year, why buy from a US Seller? wink For those not wanting to go the eBay route, I'm pretty sure Banggood sells them as well.

info device

To make it clear, before this question will surface. Device that CraigW wrote about is good for "educational" and information purpose. If you are curious how much energy your device is using at particular moment, it will give you an answer.

But it will not "repair" old or dead battery! It may help detect some problem with device showing unusual power usage, but still will not show what is wrong.

For me it would be interesting if this device could register parameters change with time, either through connection with computer or internal memory. This way you will be able to see whole cycle of charging without watching it for hours in real time smile But in future it's possible that it have this functionality.

USB Meter

They make meters that will plot the performance over time - but they cost more. Here's an example (I have not tried this model)


Great Info

Thanks for sharing ..... very interesting !

Travel Safe!

Thanks, but shop

I'm grateful to CraigW for alerting me to the existence of this product category. I've ordered one from the seller linked in the first post.

There are plenty of products in this category on offer (I suspect many are made in one town in China using the same chip). The linked seller was selling this product for $3.49 in December 2017, and is charging $8.79 plus cheap shipping as I type. So you may wish to shop a little. Not only for price.

One thing to watch for is voltage and current range. While the original USB is stated to be 5V, plenty of real implementations (especially of chargers which just use the connector format but are not full-up USB devices) put appreciably more on when not loaded. Further USB-C allows a negotiated voltage up to a nominal 20V. I'd advise checking for a claim up to nearly 10V (which the linked one has) at least.

Similarly for current, while elder days USB devices were not supposed to get very much, there are plenty of higher-output chargers out there, and devices able and eager to take more if they think it available. I'd look for 2A limit at the very least, and value up past 3.

It appears that the readability of the displays varies appreciably among the various products out there--I'd suggest reading review comments and deciding whether you think your choice is suitable to your purpose.

I'm not clear on whether all the products totalize the same way. Ideally I'd like one that separately displayed mAhr and Whr, resettable by a push button. While real-time display of current conditions is already valuable, in making a choice I'd compare the totalized display(s) to one's needs.

I'm getting old. The idea that you can buy something that does this for five bucks is just mind-boggling to me. I'll post again when mine shows up and I get some usage out of it.

I've got various questions in mind already. Just for one example: does my tablet really get higher power delivered to it when it detects that it is being supplied by its own charger and announces high rate on the display than when I plug it in to a socket capable of supplying over 2A but not delivering the secret handshake?

personal GPS user since 1992

If I understand you question...

archae86 wrote:

I've got various questions in mind already. Just for one example: does my tablet really get higher power delivered to it when it detects that it is being supplied by its own charger and announces high rate on the display than when I plug it in to a socket capable of supplying over 2A but not delivering the secret handshake?

If I'm understanding your question, I can say that my genuine Samsung Adaptive Fast charger (and my Samsung "Adaptive Fast Capable" power bank which can offer 9V to a Samsung Adaptive Fast charging smartphone and even 12V to a tablet) will pass the info between charging side and receiving side to allow the charger to offer 9V rather than the standard 5V to dumb receiving devices so I think it likely that your putting the meter device between charger and chargee will allow the high power handshake needed for high power charging, whether higher amps or higher voltage and amps. When adding the meter in the middle when charging my cell phone, it shows charging at about 9V and current from about 1.5A when starting and dropping to 0A when fully charged.

Of course, we all want to hear everyone's results and tests thought up and performed.

From experience...

Knowing that there is no memory charge in new devices, more small charging increments is m

First results from Keweisi meter

I received today my Keweisi USB meter ordered from the eBay supplier CraigW linked.

The display is agreeably readable. It is strongly backlit, quite high contrast, and has a pretty good viewing angle, so in most situations I'll be able to read it easily despite the rather small character size.

I plugged my LG flip phone into the charger it came with, and quickly the display indicated it was getting half an ampere at 5.0 volts. I clicked the (only) button so the accumulating mAHr and time would be from that moment on. I was interested to see that the timer stops accumulating when the current drops to zero.

Next I plugged my Samsung tablet into the charger that came with it (and use of which gets the tablet to report that "fast charging" is underway). While the voltage was reported initially at 5, quickly it showed the result of negotiation, and settled to about 7.5 V at about 1.5 A (so lots more power than standard USB 5V/0.5A), only in a few more seconds to move on up to a bit over 9V, still around 1.5A.

Both units were at a fairly high state of reported charge at the beginning (something like 90%), so it was reassuring to see the reported amperage drop off fairly quickly. Lithium-ion batteries need gentle handling in the late stages of charging, as exceeding the prudent voltage limit even briefly reduces their lifetime, and also is adverse as regards safety (fire...).

To get this back to the GPS topic, I intend soon to try my Garmin 3597 to see how much power it takes when plugged in to my computer USB port. My observation is that when I plug it in for map and waypoint updates, it seems to charge at a very low rate, as though the power delivered to it from a standard USB port is only a little more than it consumes internally. Then I'll try it on a port rated for 2.1A and see whether Garmin will take more when it is available--my guess is not.

personal GPS user since 1992

Anyone know if it's better to slow charge?

Thanks for the info!

I wonder if it's better to slow charge, rather than fast charge? Seems like the batteries would heat up less with a slow charge.

Depends on state of charge

Lithium-ion batteries don't like the voltage to get too high. Decent chargers control that very strictly. Indecent chargers can cut time off the time-to-full by cheating and letting the voltage get higher than it should in the late stages.

But the internal resistance is pretty low, so overheating is usually not a limitation in high current charging at low states of charge.

You want to see a taper if you are watching the current. These USB-monitoring gadgets can't see the voltage on the cell, as that is behind some circuitry. If you could see it, you'd like the cell voltage not ever to get too high. Something like 4.2 volts per cell for lithium-ion.

Now if you are talking about NiMH, there are certainly chargers out there that brag about short charging times which get the cells seriously hot. I think this is a really bad idea, and I avoid chargers which brag of 15 minute or half hour charging times for NiMH cells.

Hot tip--if it is too hot to pick up, I'd distrust the long-term effectiveness.

personal GPS user since 1992


Very cool. Thanks!


archae86 wrote:

I received today my Keweisi USB meter ordered from the eBay supplier CraigW linked...

Archae, my results mimic yours. For devices not 'quick charge' compatible, we can still see when devices and charger can use more than 500mA so we see a high-current start followed by a tapering of current as full charge is approached. Like you, I have Samsung Fast Adaptive Charge compatible chargers and devices and see the initial USB 5V charge jump to 9V (at well over 1.5A initially) which is great since it means the addition of the Keweisi midstream still allows recognition of fast charge devices on each end to allow Samsung's higher voltage method of fast charging.

Thanks for the tip

Nice write up.

Pretty cool

Always fun to watch and see changes. Thanks

Thanks for the tips. Good

Thanks for the tips. Good info.

Great info, also a good device to weed out junk USB cables

Bought a similar device on Amazon a while back to determine why my phone did not always charge at the same rate. Using the same charger with different USB charging cables resulted in vastly different charging times. Turns out that similar cables from different vendors can carry quite different amounts of current.
Generally the longer and/or thinner cables carried far less current. But even similar length cables showed different amounts of charging current. I'm guessing the wire gauge inside the insulation was not the same, even though the outer insulating jacket seemed roughly equal in diameter. The device clearly showed the difference in current. I never would have thought that differences would be as big as they were between 3 and 6ft USB cords. I bought my tester on Amazon:

Garmin Nüvi 660 & 3490LMT with ecoRoute HD, Nüvi 855 gone to GPS heaven

@ scott_dog

Depends on the battery chemistry. Some batteries can tolerate fast charging better than others. Check this site for tons of info on batteries:


Garmin Nüvi 660 & 3490LMT with ecoRoute HD, Nüvi 855 gone to GPS heaven

Great Tip

Thanks for the great Tip

Another test toy

Well, even with shipping from China, most of us who have one of these testers now have received them and played around some with them. They're a good way to see what's what with your GPS, GPS battery and your charger's voltage and charging amps.

Well, maybe now you wonder about the USB ports on your computer and your dozen or so USB chargers you have scattered around the house. What current can they provide? Here's the next toy to buy (from this or any other seller):

I did order from the above seller and got delivery in just under one month.

Combining Toy2 with Toy1, you can see what current your charger offers and judge its quality by seeing how much the voltage drops as you increase the current load. I've tested some good products like name brand Samsung, Barnes and Noble, etc. chargers and battery packs and find they are honest when they indicate that they're 1 or 2 amp chargers since they maintain close to the full 5V from 0 to the rated current. I have a couple really cheap ebay supposedly high current chargers and plan to see how well they perform next and expect less than stunning results.

In any event, here's your next potential under $5 toy that will actually provide you with some more useful information.

Thank You

Thanks for the info. Very interesting as I had missed this topic earlier.

Internal resistor drops some voltage

I obtained both of the measurement device models mentioned in the original conversation. If you peer through a transparent case, you can see a square device which I believe is the low value resistor they pass the current through on the way from entry to exit in order to measure the voltage drop across the resistor, and thus report current.

Oddly, it appears they choose to measure reported voltage up-stream of this resistor, so at higher currents they over-report the voltage seen by the device being charged by enough to notice.

You can also see that QC (Quick Charge) protocol devices such as my Samsung tablet, which negotiate a higher voltage with compliant chargers, look like they are getting more than the 9 volts they are probably sensing at the input.

If you have two of these (as I do), you can review this situation by attaching them in series, then reversing the order.

personal GPS user since 1992

Not Just For A GPS

Thanks to the OP and everyone else for the tip on this. When this thread started, I ordered one of the Keweisi meters - not to look at my GPS, but to look at my cell phone battery.

A few months ago, I bought a new battery for my old Samsung G3 phone and have been disappointed with its performance. With almost every option (Location, WiFi, Bluetooth, etc.) turned off, and very little texting or phone calls, I've been having a problem getting 10 hours on a charge. To confirm that the issue is the battery, and not some unknown reason for high drain, I decided to "splurge" on a Keweisi meter.

It was a good investment - in my eyes. The meter confirmed that going from 20% charge to 98% charge required less than 2/3 of the amp-hours that should have been needed, based upon the claimed battery capacity.

- Tom -

XXL540, GO LIVE 1535, GO 620

even more deficient

-et- wrote:

The meter confirmed that going from 20% charge to 98% charge required less than 2/3 of the amp-hours that should have been needed, based upon the claimed battery capacity.

It is worth bearing in mind that the conversion circuitry in the phone which takes USB input power and sends into the battery is not 100% efficient. It is also worth remembering that it takes more than 1 mAHr passing through a battery to charge it up by 1 mAHr. So in your cited case the battery was probably underperforming by even more.

The only caveat I'd make is that battery percentage state of charge reporting is always a bit of a guess.

personal GPS user since 1992


Leann something new today.


thanks. smile

Nuvi 2598 | Nuvi 350 | eTrex Vista | eTrex 30x

Thanks for the info

Nice device.

Check the specs before you buy.

I was a bit bored and did a little research this morning.

Some of these devices are only listed as USB 2. Not likely an issue since I think it will only affect data transfer speed. Just need to keep in mind where you use it.

Some are listed as compatible with QC 2. Since they are only measuring voltage and current. Not sure why they would still not be compatible QC 3 devices.

Some only have a voltage range of 3 to 9. That may be a problem since QC 2 and 3 can go to 12V.

Some come with dual output ports if you need that.

Also saw a model that not only has a A connector in and out. It also has C type connectors as well.

So check the fine print.