Simple low-current voltage limiter in a solar battery charging circuit

Question

I am trying to devise the simplest/cheapest possible circuit for using a small solar cell to maintain the charge on a single-cell lithium battery. The reason for low cost is that I need to build fairly large number of devices employing this circuit, and the solar cell and battery alone will probably add up to a significant fraction of the total unit cost even in the best case.

The solar cell in question is nominally 6V and produces only ~5 mA of current in full sunlight. It's that very low current output that makes things difficult. I don't want to use a larger cell if I can avoid it, due to both cost and space.

A key requirement is also that there be minimal current leakage when the battery is not fully charged, as the goal is run a very low power circuit indefinitely off the battery with only intermittent sunshine to keep the battery topped off.

My amateur's thinking is to send the solar cell current directly through the battery but with a limiter to prevent the battery from ever seeing more than 4.2V at its terminals, which is the max voltage at full charge. That requirement rules out a simple Zener diode shunt, because the knee is too soft: the battery voltage would have to drop far below 4.2V before the current effectively shuts off.

One off-the-shelf circuit I've tried is this one: 1. But it consumes all of the current from the solar cell and passes nothing to the battery, even after removing the on-board LEDs. It's clearly not intended for something that can only source 5 mA of charging current.

So what I'm hoping for is a circuit that behaves somewhat like a Zener diode and will shunt >5 mA near 4.2V but have minimal leakage at voltages much below, say, 4.0V. And of course it must not consume more than a milliamp or so of the scarce available current from the solar cell when the battery is being charged.

I am not an electrical engineer, so I don't even know whether such a circuit is achievable, but I'm imagining something involving the combination of a Zener diode and a low-power comparator or similar to convert the soft knee to a sharp threshold. Forgive my ignorance if this makes no sense.

EDIT: Some comments ask for additional details:

1. The device in question will operate outdoors and is tentatively expected to draw ~0.1 mA on average (it will be turned of completely most of the time and wake up 4—6 times per hour).

2. The proposed battery is an 18650 nominally rated at 2600 mAh. This is therefore sufficient to run the above device for at least a year without charging.

3. The solar charger must therefore be capable of replacing the ~0.1 mA used by the device averaged over the entire year. It is expected that the bulk of the charging will occur during the sunnier summer season and that a charging deficit will prevail during the winter.

4. The solar cell I'm trying out is nominally 6V (approximately 6.3V actual for I < 2 mA or so). I don't have a datasheet for it, but I've made my own measurements of I vs. V for less-than-ideal sunshine. When connected to my battery (or any other moderately conductive load), it behaves more or less like a sunlight-dependent current source of up to 5 mA. This is a factor of 50 above the expected average yearly demand. If its output averaged over a year nevertheless proves inadequate, I'll go to a larger-area cell, but I'd prefer to squeeze as much efficiency as possible out of the smaller, cheaper cell before giving up on it.

5. Expected temperature range most of the time is –10°C to +30°C, though it may very occasionally get much colder (–20°C or even —25°C).

6. Location is in the U.S., initially in Madison, WI, at 43°N, 89°W, but the device is intended for use at other locations in the future. I do have the ability (with moderate effort) to evaluate available solar energy as a function of location and date, but again, it's the yearly average that matters giving the battery capacity.

7. Yes, I can program a microcontroller, but I really don't want to put a second one on the device, and the first one will be powered off most of the time.

Answer 1

The following should go a long way towards an acceptable answer.
For an optimum answer you'll need to supply details such as LiIon cell capacity, charging environment (inside/outside, geographic location(s) temperature range), ideally panel spec (web-link and/or Voc, Vmp, Imp, Isc) ideally panel type - and more.

What you are asking for is in fact a voltage limiter.
If you set this to 4.2V, then if this voltage is reached for more than a few minutes per charge cycle then the cell will be destroyed rapidly - and possibly in a thermally exciting manner.
LiIon cells MUST not be floated at full voltage.
Metallic lithium may plate out and this may lead to a "vent with flame" event.

If you wish to charge to 4.2V then you must use a CCCV charging cycle or terminate charge at 4.2V. Either adds complexity and cost. Fortunately, there is an alternative that is probably acceptable.

Floating at slightly less than 4.2V - say 4.0 to 4.1V, is acceptable.
This reduces cell capacity somewhat. As you have a maximum of 5 mA charge, then if this application is an outdoor one summer charge hours of full equivalent sun are usually in the 5 to 6 hour range. And winter equivalent full sun hours are typically about 2 hours per day. These duractions vary in extreme locations - northern areas with snow will usually be mess. Moscow Russia manages about 20 minutes equivalent full sun in mod winter.

Assume SSH (sunshine hours of equivalemt full sun) are in the 2-6 hour range.
Battery charge per day in winter is about 10 mAh and in summer about 30 mAh.

If outdoors, and if able to run only on solar in winter you need only 10 mAh.

If you need multi day holdover these capacities will be multiplied by the number of days of holdover.
eg being able to handle 5 sunless days in winter requires a 5 x 10 mAh = 50+ mAh cell. In summer this would be charged in 2 days. A 100 mAh cell would probably meet needs unless costs are exceptionally tight.

---

Zener solution:

IF you can tolerate a substantial reduction in cell capacity then using a zener diode clamp such that the soft knee never allows the cell to charge above say 4.1V absolute maximum MAY be viable. Cell capacity would probably be 70-80% of nominal. This may be acceptable as if your application is an outdoor one then winteroperation will only give typically 10 mAh of capacity. A say 100 mAh cell would easily tolerate the resuced acapcity.

---

Sharp cutoff 4.1V clamp - TL431 (was TLV431)

Edited:

• After looking at the TLV431 supply situation I looked at TL431's. These proved to be in stock from MANY suppliers via LCSC and prices are very low compared to the TLV431. In 1000+ quanity some brands are under $US0.03 each and some "Western brands" are under $US0.08 in 1000 quanity eg here.

• The TL431 will allow a very low quiescent current clamp to be implemented if a 2 transistor (maybe 1) driver is made to disable the clamp when the panel is not providing current.

Asian brands are potentially very acceptale - as long as due diligence is done.

---

A potentially suitable candidate for a voltage clamp is a TL431.
These cost a few cents each in large volume from China. LCSC is a resputable Chinese supplier - usually stocking these from many manufacturers. As can be seen here most of these are currently non-stock. Prices are from under $US0.02 each in 3000 quantity.

The TL431 is a programmable zener with a 2.5 minimum clamp voltage which can be increased to any desired value using a resistor divider.

A TL431 has a minimum regulation current of about 1 mA but until the cell reaches the desired clamp-voltage current drain during operation can be maybe 50 uA due to current drawn via the resistor divider. At say 50 uA and 5 mA charge that's about 1% power loss.

In its most basic form the TL431 always draws current from the cell. This would result, at 50 uA, of loss of say (50 uA x 24 hours)/10 mAh or about 12% of a 2 hour day's charge overnight!

Non charging loss can be reduced to about zero by the addition of a circuit which turns off the TLV431's reference divider when the solar panel is not operating. This can probably be achieved using 1 or 2 transistors and a few resistors.

Total voltage clamp component cost in volume from a reputable Chinese supplier is probably around $US0.10. A custom microcontroller solution would allow extra features to be added for little or no capital cost but it would be hard to match this pricing.

I'll add a circuit diagram "soon".

---

The TL431 is made by a number of manufacturers.
Some Chinese sources are very reputable, some aren't. Start with data sheets from known quality manufacturers for design purposes and then investigate supply options.

Nexperia

Datasheet

---

OUTDOOR USE

If this is an otdoors / sunshine powered application then the Gaisma website will provide invaluable information about solar insolation.
As I do not know your location I chose Denver, USA as an example
The Gaisma page for Denver here shows TYPICAL SSH (sunshine hours = kWh/m^2/day = equivalent full sun hours) of about 2 hours in winter and 6 hours in summer.
Find the Gaisma page for your location(s)

---

Suggested TL431 voltage clamp.

Operation:

TL431 is set to clamp voltage by R1 & R2.
When the PV panel is making voltage Q1 is turned on by R4.
Depending on MOSFET chosen R4 may not be needed (= shprt circuit).
Adding R4 decreases sensitivity to low level light. The circuit needs to be designed so that Q1

• switches off in low light
• starts to turn on as Vpv approaches charge voltage and
• is fully on during charging.

D2 is a low reverse leakage diode.
D2 forward voltage drop is non critical in almost all cases as VPV_OC will be substantially higher than Vbat_max.

When Q1 is on the TL431 is connected across the battery and shunts current when Vbattery is >= Vclamp.
If batteries pre-charged to above Vclamp are ever to be inserted a resistor is series with the TL431 is essential so that Idischarge does not exceed TL431 maximum.

In low light Q1 is off and the TL431 and its bias resistors are disconnected.

---

Vclamp is set by R3 and R2.
Vclamp ~= 2.5 x (R3+R2)/R2

• Positive voltage clamp voltage error is caused by reference bias current (about 1 uA typical so with values shown about +0.06V per uA positive clamp voltage error)
  Less reference error due to r2 current can be obtained with lower R2 and R3 values, at the expense of more loss when not clamping.
  As shown R2+R3 drain about 40 uA from the PV panel durung charging - unlikely to be significant.

FET Q1 should be turned on by PV panel when charging and turned off due to too low Vgate when not charging.

The FET Rdson resistance notionally causes increased clamp voltage when clamping but this should be negligible.

• due to V = I x R. For say 5 mA clamp current and 0.1 ohm Rdson
  Verror added = I x R = 5 mA x 0.1 ohm = 0.5 mV = negligible.

^{simulate this circuit – Schematic created using CircuitLab}

E&OE.
Queries welcome

Answer 2

Extremely educated & useful answer from @Russell McMahon !! As a simpler alternative (at the expense of lower kWh density), I would suggest 3xAA niMh (plus a diode)...