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Solar Chargers, battery packs and Cell Phones
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ArticleId: 4076 Written: 2013.09.28 by: Robin Tivy

I'm starting this discussion so we can all learn a bit more about external battery packs, solar chargers and cell phones. Providing your cell phone with additional energy mid-trip is important especially when you use it for navigation or track logging on longer trips. There are quite a few poorly designed battery packs and solar panels on the market, and some vendors deliberately try to mislead the consumer by quoting the capacity in mAh rather than Watt.Hours. So my objective here is to provide some background theory, and then describe the experience we've had with a particular unit.

As you know, there are two ways to get extended power into your phone on long trips:

  1. Recharge it from an external battery pack
  2. Use a spare phone battery

The units we have experience with are the Samsung Galaxy S4 phone, and the "PowerMonkey solar charger" available at MEC for $90. But I'd like to hear about other models as well.

Note that both the input USB port (in the phone) and the outbound USB Port (on the battery pack) are not simple electrical connections - they are like little computers that talk to each other and set the charging rate, and prevent overcharging, etc. As evidence of the complexity, see The Basics of USB Battery Charging: A survival Guide.

In this article, I don't get into the handshake logic, instead I focus on a more basic understanding - just comparing the energy capacity of the battery pack versus the phone battery. How big is the battery pack versus the internal battery?

The most convenient unit in which to measure energy capacity is the watt-hour. If a battery has a capacity of 10 watt-hours, it means it can put out 10 watts for one hour. Or 1 watt for 10 hours. Here is a link to a powerful 44 Watt-Hour battery pack with the specifications stated correctly. PowerStream MP3500 Specs.

In the past, most AA batteries only advertise their capacity in milli amp hours, instead of being in the proper units. A typical AA battery has an output of 2000 mAh at about 1.2 volts. So to get the capacity, you multiply the volts times the amps. That gives you 2.4 watt-hours. So that big unit above stores the same energy as 18 AA batteries. Once you've got the capacity in watt-hours, you can ignore the voltage. All these battery packs have voltage regulator circuits in them which can output different voltages. If the voltage is higher, the total current available must be lower. Eg: The 44 Watt-Hour supply could put out 1 amp for one hour at 44 volts.

Many battery pack manufacturers try to advertise their capacity in terms of milli-amp hours or mAh rather than watt-hours. This can be really misleading. For example, I saw one that contained only 3 AA batteries advertised to be 7000 mAh. (they just added up the mAh of the three batteries, at 1.2 volts). And then somewhere else in their spec, they said it had an output voltage of 3.8 volts. So at first I, thinking its capacity was huge: 7000 times 3.8 volts, when in fact it was 1/3 of that.

The Samsung Galaxy S4 has a 2600 mAh lithium ion battery, which runs at 3.8 volts. The PowerMonkey battery is 2200mAh, at an output voltage of 4.5 to 5.5 volts. So the capacity of the power monkey with a slightly higher voltage but smaller mAh is probably close to the cell phone. Here is a summary of the specifications:

  mAh volts Watt-hrs Weight -----------------------------------------------------------
  Power Monkey battery 2200 4.5 9.9 77g
  Samsung Galaxy S4 2600 3.3 8.58 44g
The solar panel itself is an additional 90 grams.

In order to cause charge to flow from the battery pack to the phone, the battery pack must be at a higher voltage. In this case, 4.5 volts versus 3.3 volts). So if you hooked the two together, I assume the charge would flow until the battery pack was almost empty, and the phone full. I'm assuming the charger logic would maintain the 4.5 volts even when the charger is almost empty, and there would never be backward flow. (This may not be completely true. I read somewhere that one of the battery packs had an 88% efficiency.

A spare battery for the Samsung Galaxy S4 is available in Vancouver for 45.00 (lists at $39.00 USD) on the Samsung website. The battery is easy to replace, just pop the back off the phone.

EXPERIENCE: Here's Betsy's experience on our recent Powell Divide trip: (with a Samsung Galaxy phone and a PowerMonkey battery pack.)
 She started out with the phone and battery pack fully charged. She then used the camera and GPS function numerous times over the next four days. Then on day 4 we were stuck in the tent, the phone was down to 1/2. She took out the battery pack and charged the phone back up to close to 100%. This took the battery pack down to probably half. She then deployed the solar panel inside the roof of the tent to recharge the battery pack. It seemed to raise the charge one or two "bars".

The night before the last day, she charged the phone to 100% which completely flattened the battery pack. The last day was bright sun and she deployed the solar panel on the outside of her pack for 7 hours as we came over the top of Danelaw Peak. Meanwhile she turned the phone on for an hour or two at the high point to attempt email transmission, which drained the phone down to 50%.

At the end of the day, the battery pack was up to 4 bars. (say half). She then charged the phone a little bit, raising it to 60%.

Based on this experience, we'd say that the solar panel itself would take at least 2 days of clear sun to fully charge an empty battery pack. Her experience in June in Europe would suggest the same thing. On solar power alone, the system gradually works its way down, and she would have to recharge from the wall plug.

Her theory is that it's better to "top up" the phone as soon as it goes down a bit, and then start gathering energy, rather than running the phone way down before topping it up.

Anything that can be charged with a USB cable can be charged with the battery pack.


#1830 - 2016.04.08 Steve Grant - Buying Replacement Phone Batteries
Robin asked how you can tell the clones from the oem batteries. It's really important to ensure you get oem batteries. The clones usually work well for a month or so, and then basically quit holding a charge.

Probably batteries sold by local bricks & mortar stores will be genuine oem, but they usually have huge markups.

Batteries sold on Amazon are more likely to be the genuine article than those sold on eBay. My impression is that Amazon polices bs better than eBay.

So let's try eBay anyway. First search is for (samsung s4 battery). Over 6000 listed. Do another search for (samsung s4 battery oem). 600 listings.

Some specify the US or Canada as point of origin. This doesn't mean much because the seller may be getting cheap clones from Korea, China or Hong Kong. So far as I know, none of these batteries are made in North America anyway.

Next, choose a few and look at the largest, most detailed photo of the batteries possible. Compare them to your own battery. The slightest difference in symbols, font size, spelling, layout etc will indicate a clone.

Read the ad text and reject any that are illiterate, have low numbers of sales, or more than very few dissatisfied buyers.

Beware that some sellers use the words "oem" or "factory" or "genuine" or "original" or "authentic" to mean only that their clones can be used in place of the original battery.

In general, the more expensive the battery, the more likely it is to be genuine oem. But you can also waste more money if it turns out to be a clone.

Without checking the graphics on the battery, here's an ad I would tend to trust:

"Original equipment battery manufactured by Samsung"

#1829 - 2016.04.08 Robin Tivy - MAh is not valid units for capacity comparison
Thanks to Bill Leach for providing some more data on actual real life battery pack specs. What we need is more such postings, with real life data.

I still think the best way to compare energy capacity is to use the proper units for energy capacity. The proper units are watt-hours (wH). If the manufacturer is giving the capacity in the wrong units they may be wrong (as I noted in the model with the 3 AA batteries). The only reliable specification is when the manufacturer gives the capacity in the proper units, which are joules or watt-hours. Then you know the person writing the specs at least knew the basic concepts. For example, the battery that ships with Samsung S4 is labelled as at 3.8 volts, and 9.88 Watt-hours. They don't talk about milli-amp hours.

When comparing a strategy using external battery packs to replaceable batteries, you also have to take into account the transfer loss. For a given weight of lithium battery, you are always going to lose some energy doing the transfer. So it's better to have the energy in the phone batteries to begin with. With an external pack, there are two voltage changes that must take place:

1. From internal battery pack at 3.8v to USB voltage of 5.0 v
  2. From USB 5.0v back down to phone battery 3.8v

A cell phone engineer I talked to estimated there could be 10-15% loss on each transformation. And even more loss at higher voltages. Both of these conversions are avoided with a removable battery. And changing a removable battery is instantaneous, as opposed to having to recharge it overnight while on your trip. Many times I've seen the low battery warning after a day and a half of track logging, and I've popped the back cover off and swapped in a new battery, and then continued track logging. With a battery pack, I'd have to wait till I was in camp to recharge. Or recharge every night.

I acknowledge that eventually the removal of the back of the phone to swap batteries might cause problems. But it hasn't been a problem. We've been doing it for a couple of years with our two Samsung S4 phones with no sign of a problem.

I would be very interested if anybody does an actual experiment of seeing how many actual charge ups they can get out of a given battery pack. For example, suppose the battery pack is rated at 40 watt hours, can it completely recharge a 10 watt phone battery four times? One way to do the experiment quick would be to recharge multiple dead batteries after a long trip. One review of the patriot said that the +9000 model would only be good for 2 recharges.

 Samsung battery weights 44 grams. The battery that ships with Samsung S4 is labelled as at 3.8 volts, and 9.88 Watt-hours.

I couldn't find any capacity specs for the Patriot battery pack Bill mentions.

#1828 - 2016.04.08 Bill Leach - My experience with an external battery
First, given that many external battery packs are built for USB (5 volt) output, I think the mAh rating does allow a direct comparison between phone batteries and external battery packs. For example, my phone battery is rated at 1630 mAh and my external battery is rated at 7800 mAh. I would expect to get four good recharges from the battery pack. Remember that batteries depreciate over time and leak, so you will not get the full rated capacity.

I use a Patriot Fuel external battery that weighs 200 g. I see that Amazon has a newer Patriot Fuel 9000 mAh battery with the weight stated as 180 g.

On a recent four day trip I was able to use the phone to GPS track each day's travel and do a bit of reading at night, then recharge the phone at night.

I would prefer an external battery pack over extra phone batteries as they are easier to charge, don't require taking the back off the phone (with eventual breakage) and can be used while the phone is turned on. Moreover, my phone battery weighs 40 g, so four spares would weigh as much as an external battery.

#1827 - 2016.04.07 Robin Tivy - If you google "Samsung Galaxy S4 battery" you find lots for under $10.00
I can't figure out if the batteries in Amazon are truly the originals, or are fakes. Please google "Samsung Galaxy S4 battery". You will see about 10 choices of companies. At least 3 or 4 look to be the identical OEM battery. We read the reviews, which are mixed, but mostly about disappointment with the performance, which I suppose could happen even if they were original. Many of them show a photo of an actual original Samsung battery.

So here's the question: Of these two listings in Amazon, what is the company? What is there in the part numbers that tells you anything?

  1. One that is 35.15

  2. One that is 7.35
What is the difference?

Previous to this posting, both Betsy and I have bought spare batteries from Vancouver Battery for about $40 and they seem to perform the same as the originals.

#1721 - 2013.10.03 Robin Tivy - Specs for more battery packs
I went into the battery store on broadway and looked at what Lithium Ion battery packs they had for sale. The main two were by "Lenmar.

The bigger of the two Lenmar was advertised at 7000mAh. By scrutinizing the package, I was able to see its capacity stated at 25 watt-hours. It weighted 0.45 lbs. So that would have 3 times the capacity of the Power Monkey.

#1720 - 2013.10.03 Robin Tivy - Recharging the phone from the battery pack
Here's the latest experiment: Betsy ran the phone down to 27% of its charge, then tried to charge it back up with a fully charged Power Monkey. It was only able to bring the phone up to 84%.

According to my capacity calculations, the battery pack should have had enough charge to completely charge the phone. It has 9.9 watt-hours trying to charge an 8 watt-hour battery. But it didn't.

So this suggests one of the following:
 1. all the capacity in the battery pack is not truly available
 2. There was loss in the transfer.
 3. The specs are wrong.

They never really directly state the watt-hours on the MEC website, they only state the mAh in one place, and the output voltage somewhere else. It is entirely possible it can't actually put out that much current at the stated voltage.

#1719 - 2013.09.29 Robin Tivy - Comparison of Battery Packs versus spare phone batteries
Steve's posting provides exactly the type of info we are trying to gather. It sounds to me like the "Tech Charge" is really just a battery pack, whereas the Power Monkey has more sophisticated electronics. Certainly on the Power Monkey, at no time is there the possibility of the battery pack draining the phone. This is the first thing you should check when buying one of those units. It is strictly one way. Probably the battery pack has circuitry to prevent backward flow. It has indicator lights that show when it is charging, and also the level of charge in the unit.

The weight comparison point is also important. The Power Monkey battery pack weighs 77 grams, and a spare battery weighs only 44 grams. So you could carry only two spare batteries for the weight of the battery pack. The power monkey battery pack is Lithium Ion, rather than being something that takes NiMh batteries. The solar panel itself weighs an additional 90 grams, so that would be the first thing I'd leave behind.

Steve didn't give the weight of his phone battery or the Tech charge, but this would be useful.

Another point in the comparison between a spare battery strategy versus intelligent battery pack is the ability to use the battery pack to charge other devices. On our recent trip, we also used the Power Monkey battery pack to charge my Sony E-Reader. It charged it up in just a few minutes. It will charge anything with a USB connection.

Frank (who works for blackberry) explained the tradeoff of removable batteries in phones. He said having a removable battery adds 1 mm of width to the phone, and perhaps 10 grams of weight to the phone. And makes the phone a little less stiff. This is the reason some manufacturers such as Apple do not have user accessible batteries. But for mountain purposes, I'd still choose a phone with an accessible battery, just to prolong the life of the phone.

#1718 - 2013.09.28 Steve Grant - Tekeon TekCharge
I've used a Tekeon TekCharge. It's a battery pack that holds 2 or 4 AA cells, and charges phones etc. using a USB-type connector. It can also be used to charge those batteries via USB connection to a computer.

What I found was that whichever device, the phone or the TekCharge was at lower charge, the connection would try to balance them. So if the phone was at 75% and the TekCharge's AA cells were drained, the TekCharge would actually, and rapidly, drain the phone until the charge level in both devices was balanced. Which means that the TekCharge is perfectly usable, but after its batteries are drained a little, you cannot charge the phone to a higher level than the charge in the AA's. So gradually the phone will reach only a lower and lower charge level.

I did this with rechargeable NiMH cells, which provided the equivalent of about 2 full charges of the phone. Probably premium disposable AA's would provide several more charge cycles for the phone.

Instead, I carry a spare phone battery. I could carry a dozen of them for the same weight and volume as the TekCharge.