Trickle / float charging a car battery from a PV panel: questions

Re: Trickle / float charging a car battery from a PV panel: questions

Welcome to the Forum!

inschris wrote: »

a) Information is that, for such battery,:
i) the self-discharge rate is between 3% and 20% per month. Assume a self-discharge rate of 10%; for a 40Ah battery that is 4Ah per month, ie about (4Ah/(30.5days*24h) = 5.5mA (?average).

OK numbers to work with... A couple of issues, while the "float" current would be 5.5 mA average for 24 hours--You have limited amount of useful sunlight per day--So--assuming "non-winter" usage--A good number to start with would be ~4 hours of "noon time equivalent" sun per day.

Next, you can pretty much assume that a panel will only operate, on average, around 80% of rated power (off angle, dirty panel, etc.)... So, the actual panel current rating would be:

• 0.0055 Amps * 24 hours per day * 1/4 hours of sun per day * 1/0.80 fudge factor = 0.04 amps

Now--if you can find a "6 volt panel" rated at Vmp~7.5-8.5 volts, then the wattage rating will be:

• 0.04 amps * 8 volts = 0.32 watts

But, if you cannot find a 6 volt panel--12 volt panels are more common, so you would need a 2x higher wattage rated panel of 0.64 watts--Still a very small panel.

Since it can be difficult to find off-the-shelf 6 volt solar charge controllers--I would suggest getting a "12 volt panel" and installing a ballast resistor to limit the charging current. Also, a typical maximum "unregulated" charging current would be around 1% of battery AH rated capacity... So those numbers would look like:

• 40 AH * 0.01 rate of charge * 17.5 Vmp panel * 1/0.8 derating = 8.75 watts @ Vmp~17.5 volts
• 40 AH * 0.01 rate of charge = 0.4 amps float current

And, assuming it gets hot in the summer, your ballast resistor would be rated at (note, hot solar panels have lower Vmp--Assume ~15 volts minimum in summer):

• V=I*R; R=V/I
• R = (15 volts Vmp - 6.9 volts float) / 0.4 amps float current = 20 ohms
• Power = I²R = V²/R = V*I = 0.4 amps ² * 20 Ohms = 3.2 watt minimum rated resistor (or heater, or filament lamp, etc.)

In theory, 1% or less rated current solar panel would not need a charge controller... And once the battery is charged, you really want to float it at a reduced voltage to keep the water from being electrolyzed into hydrogen and oxygen (and needing more distilled water every month or so). For a 12 volt battery, "float" at ~25C is around 13.2 to 13.6 volts (ideally). 6 volt battery would be 6.6 to 6.8 volts or so.

So, if you set up this system, you will want to monitor it (water level, state of charge with hydrometer, charging voltage etc.). Hot batteries discharge faster than cold batteries--And you have more sun in the summer--So you may want to change the resistor value higher or lower based on your observations.

You do not want too much current--besides driving the water out, the excessive "equalization" (generation of gasses) does cause internal aging in the batteries (shedding of materials, positive grid corrosion, etc.). Above ~6.8 volts, you are now actively charging the battery and generating these gasses, driving water out, etc.--Which is not really helpful for long term "storage".

ii) Continuous-preservation (float) charging voltage is 6.9V or thereabouts for flooded cells

Probably a bit high... But not the end of the world.

b) At the same time, consider a solar panel nominally 6V 1.6W; this is, allegedly, down in the trickle- or float charge range for a 40Ah battery, and can be left permanently connected to the battery, without any regulator (still needs a diode). It appears that this panel will have specs something like*:
Peak output power: 1.6W
Voltage (@max. power): 8.7V
Current (@max. power): 1.6/8.7 = 184mA
Open circuit voltage: 10.5V
Short circuit current: 300mA
Tested under standard condition (STC): AM1.5, 100mW/100cm2 @25°
[Does Maplin mean 1000W/sq m ?]

Yes... You got everything "right".

QUESTION 1: Is there a contradiction here; can the panel really be left "permanently connected .. without regulator"? In the mountains we get many complete days of high irradiance, low temperature and clear air. Am I mistaken in fearing that:

i) at (6.9V per b) + 0.7V for the diode) = 7.6V, the panel at STC will still push around that 184mA into the battery, or more, way beyond that 5.5mA per a)i)?
ii) the current into the battery will not fall to around that 5.5mA per a)i) until the panel and battery voltage are close to the panel's open circuit voltage, 10.5V?
iii) ie, damaging overcharging will occur, without some regulation?

If I am mistaken, where is my misunderstanding?

Yes. Many dedicated solar products with the "right" solar panel will use "natural" regulation--Around Vmp~7.5 volts for a 6 volt battery. If you can find such a panel--great. But if not, use the ballast resistor calculation above... Cheap and "adjustable" if needed.

By the way Volkwagen (and probably other car importers) have small ~3-5 watt solar panels that are used when shipping/storing cars prior to sales... In the US, you can find them on EBay--Might be a good source of small/cheaper solar panels or at a local import dealer (you would need to install under glass--I doubt they are rain/weather proof panels.

QUESTION 2
If the unregulated scheme is indeed problematic, can we use a simple regulation as in diagram solar01?** (herewith, solar01.jpg; hope it will display OK)
Ie, am I correct in expecting that, in such arrangement,:
i) the panel's output current will be practically entirely drained to earth thro the zener once the battery voltage reaches 6.9V, so that the battery voltage gets no higher? (and the panel output voltage sticks at 7.6V, w output current <300mA) It doesn't matter that this power is wasted, in the zener; there is no load to supply until we next run the engine.
ii) if the battery is less than fully charged, when first connected to the panel, then some current up to perhaps 250mA will initially go into the battery; that will bring it up to full charge within a few days (ie bring things to the situation of QU 2 i).

**with acknowledgment to http://jaw.iinet.net.au/projects/solar.html

All good wishes

Yes, it could work. Although a standard 3-T adjustable Regulator (3 Terminal Regulator) would be better/more stable. They are cheap and only need a couple small capacitors and a small variable resistor (or two fixed value resistors) to function. Plus a 3-5 watt (or less) heat sink (any energy not going into the battery is going into the 3T Regulator heat sink). I would trust it more than a Zener diode setup.

Here is one common 3T Regulator family.

Remember, batteries are temperature sensitive... Higher self discharge when Hot (try to keep battery as cold as possible--I.e., on floor or even "underground" if this is shed that gets lots of full summer sun). And hot batteries need lower charging/float voltages. A very cold battery will not need much float current. And a fully charged lead acid battery will go way under -40^oC without freezing/ill effects (other than less current / AH capacity).

Hope this helps.

-Bill