This page will be updated when I have new information
2014-08-27 Review section added with voltage/current measurements and overall impressions of product.
2014-08-31 Added max cell length
2014-12-04 Updated max current delivery upward to reflect the results of new tests with different testing equipment. Also added note about overheating cut-out under >2A load.
2014-12-09 Updated to reflect that both ports use the Apple standard to signal that devices may draw up to 2A.
I’ve been using a Tomo V8-4 power bank, or USB charger, filled with four 2600mAh Samsung 18650 cells I scavenged from a cheap, unused, laptop battery, to charge my iPhone and iPad when I’m low on juice and away from a power outlet.
There are cheaper four-cell DIY USB power banks available. I chose the Tomo V8 because it reports the charge level, and manages charge and discharge of each cell separately, which is useful for scavenged cells. The same product is available as the Soshine E3. There is also two-cell versions called the Tomo V8-2.
Review & Testing
The TOMO V8-4 power bank feels solid and well made, but it doesn’t feel like a premium product. The volume is large enough to hold ~8 cells, rather than four, which may be an issue if you are space constrained. The weight though, is reasonable. It fits four 18650 cells. I estimate that it can take cells up to 68.5mm length, and probably 69mm.
The power bank is turned on by pressing a button on the back, next to one of the USB outputs. It also powers itself up automatically when when a USB device or charger is connected.
When powered on, the display indicates the approximate level of charge of each individual cell.
USB Power Output
When powering USB devices, the right-hand side of the display indicates which port(s) are drawing current. The representation of each port has an animated indicator line that sweeps from left to right to indicate current flow. It isn’t clear to me whether or not the speed of the animation hints at actual current.
I’ve had some challenges when trying to measure current delivery to USB devices.
When used with a resistive 5 or 2.5 Ohm load, which should draw 1 or 2 A, respectively, at USB voltages (5V), the powerbank delivers current, but behaves strangely. The display shows intermittent or non-existent current draw through the port the load is connected to, and will power itself down after a time-out period as if nothing was connected.
When the load is connected to the 2A output though a USB voltage and current meter, it shows a draw of ~0.95A @4.88V with a 5 Ohm load and ~1.9A @ 4.8V with a 2.5 Ohm load.
On the 1A output, though a USB voltage and current meter, it also shows ~0.95A @ 4.88V with a 5 Ohm load. With a 2.5 Ohm load, it cuts the output and the display shows a fault condition in the 1A port.
With both ports, in use, it seems to be able to deliver a combined 2.5A without too much voltage drop, but after 5 minutes or so, it cuts out. The display backlight goes black, but the display remains powered and shows that the cells have ~50% charge. The cutout seems to be due to overheating, because it automatically resumes after a few minutes.
The bottom line seems to be that the TOMO V8-4 can provide USB devices over 2A of current at USB voltages from its internal batteries. It signals iOS devices that they can draw over up to 2A of current.
The powerbank shuts itself off when all the cells drop to ~3V.
USB Charge Rate Signalling
When connected to a computer, USB devices must go through a negotiation procedure before they can draw more than 500mA power. In order to safely provide higher charging currents using a “dumb” wall adapter, Apple created a simpler signaling mechanism for the iPod, which they further adapted for the iPhone and iPad.
Some device makers imitated Apple’s approach to maximize compatibility of their devices with chargers designed for Apple devices. However, more recently, a USB charging standard was created and has been adopted by most device makers other than Apple.
When I used the TOMO power bank to charge iOS devices while using a USB meter to check the charging current, I had confusing, inconsistent results, I decided to take a closer look at the charge rate signaling used by the USB ports. I found that both ports signal that an Apple device can draw up to 2A of current. This could cause a problem, because the port labeled as 1A will turn off the power output if a device draws 2A.
I do not know how non-Apple devices will behave when charged from the Tomo.
USB Power Input / Battery Charging
When charging, the cells being charged are animated to show that they are receiving current. The speed of the animation doesn’t seem to give any indication of the current charging current or voltage, it either runs, or it doesn’t.
The Tomo V8-4 can recharge cells while also powering USB devices, provided the USB device current draw isn’t too high. When the USB devices draw in excess of ~700 mAh, the power bank stops charging and just powers the external USB devices until they are disconnected or stop drawing current.
The largest current-draw I’ve observed when charging 4 cells is ~1.5A, 25% less than the max charging capability of the charge controller ICs (see below).
The charging section seems to operate appropriately in that current is reduced once the cells reach their target voltage and charging is stopped once the charging current drops off significantly, and doesn’t begin again until the cells are discharged to ~4V. So, it approximates appropriate Li-Ion battery behavior, but I can’t say how precisely.
I’ve observed that the two slots on the left charge the the idea 4.2V level, while the two on the right only charge to ~4.16-4.17V. This is within the 1% error on the datasheet for the charger ICs, but the grouping seems strange, particularly since someone else reports a similar pattern with their TOMO V8-4 powerbank.
I really wanted to love this power bank, but it comes up a little short.
- Charges and discharges each cell independently
- Protects cells against over-discharge and incorrect insertion (reverse polarity).
- Respectable charging behavior, doesn’t overcharge cells.
- Attractive design and solidly made
The not so good:
- Doesn’t charge iOS devices at their maximum rate.
- Leisurely charge rate for 18650 cells (~300-400 mA, max)
- Capable dot-matrix display isn’t used to its full potential. Would be useful to give numeric estimates of voltages and currents.
- Variable charge levels due to design or manufacturing issues.
After using it for a while, I decided I wanted to know what was inside. The case design is quite elegant. It feels solid, and yet it is easily disassembled without a screwdriver. I used a small plastic lever to “derail” the sliding cover while I slid it shut. With the cover off, I was able to disengage the bottom of the case from the center section.
It was easy to slip the LCD display loose from the case. From there I could tip the circuit board out. In order to release it fully, I had to compress the wire springs that formed the negative-contact for the batteries so that they would slip through the openings in the plastic case.
I don’t know about the actual design of the circuits, but the layout of the printed circuit board seemed elegant and straightforward.
The top side is primarily devoted to circuit routing, and mounting of the USB connectors and battery contacts. There is also a microswitch for turning the unit on and off, an inductor and a large capacitor, presumably for the output power regulation circuit, and a capacitor and an SS32 Schottkey diode from MIC.
Most of the components are mounted on the bottom-side of the PCB.
Voltage Leveling and Reverse Polarity Protection
There are four SS32 Schottkey diodes from MIC, each associated with the negative contact for one of the lithium cells.
I think these serve to make sure all the cells maintain a similar voltage level during discharge, and that when cells have dissimilar voltages, there aren’t large currents between them that could damage the cells.
This design mitigates some of the problems with using dissimilar cells in parallel, but not all of them. Depending on their charge levels and voltages, the current demands on cells will vary. Its possible that with a wide enough difference among cells, the highest voltage cell carries the entire load, while the other cells sit idle. Depending on the other accommodations of the design, this could lead to cells being drained at levels in excess of their design limits.
There are also four small, identical 6-pin ICs, each of which appears to be associated with an individual cell. The markings are “57bB.” My assumption, given that their are four of them, and their position in the circuit, is that these are purpose-built Li-ion charging ICs. Based on that, the markings, and the form factor, there are a variety of candidates, but I’m pretty sure they are TP4057s in a SOT-26-3 package from TP Micro.
On paper, at least, these sound like capable charging devices.
- Adjustable charging current up to 500mA
- cv/cc charging profile
- Automatic charge termination at C/10
As noted in the review section, above, as designed/manufactured, the power bank doesn’t seem to be using these to their full potential. Actual charging current seems to be closer to 300 mA.
- TP4057 Datasheet (full, Chinese)
- TP4057 Datasheet (partial, english) Note: This is for a higher-amperage version in an 8-pin package.
USB Power Output
Next, I noticed two identical 8-pin ICs, each associated with one of the two USB ports for powering external devices. They are both marked “9926A TF407C.” The 9926A marking suggests that are dual N-channel MOSFETs, but the exact specifications and origin are unclear. My best guess is that they are made by a mainland Chinese company. I don’t know enough to know exactly what function these serve. My guess is that they switch the output on and off, and might go so far as to regulate output current to manage power distribution when two USB devices are connected.
I haven’t tested this for myself, but from this Russian review on YourTube, it appears that the current limits on the outputs are managed separately.
Switch Mode Power Supply for Output
There is another IC that seems associated with the output, a single 8-pin chip. Its marked “MT5032A G349T1,” which unambiguously identifies it as a MT5032 800kHz Synchronous Step-up Converter. This forms the core of a boost converter, which is type of switch-mode power supply. This takes the 3-4.2V provided by the batteries and converts it to the regulated 5V voltage fed to the USB devices.
Some details from the datasheet:
- 2.1A output with a 3V input, at higher input voltages, it can deliver higher currents.
- ~94% efficiency at 500mA output throughout input voltage range
- ~90% average efficiency at 2,000mA output over input voltage range
- Fixed 5.1V output when pin 4 is wired to ground (as it is in this device)
As noted above, this power bank doesn’t seem to approach the theoretical output currents for this device.
The highest current observed has been 2.25 A when passing-through power from a 5V USB power adapter.
The largest chip is a PIC16F1933 microcontroller. This drives the display, which provides information about whether relative charge-level for each cell, and output through each USB port. When an external USB power source is connected, it also indicates whether or not each cell is being charged.
These functions clearly require that it have a way to read whether current is flowing through each USB port, the voltage for each cell, and whether or not each charging IC is active.
It probably also plays a role in detecting when a device is connected, turning the output ports on or off, and turning the charging ICs on an off. It could even play a role in regulating charging rate, and the distribution of output current between the two ports. I may try to figure these details out in the future, but learning more is going to require some combination of tracing out the circuits in more detail and/or probing voltages during operation.