Introduction: Solar PV Charge/discharge Current Monitoring

About: Retired educator/writer

NOTE ( Feb. 2013) : This Instructable is now dated, but has been retained for convenience. Some approaches considered here are now  poor practise. Refer instead to a fresh Instructable =>

"Backyard" photovoltaic solar setups increasingly abound, but they usually run "blind" with charging performance often unknown.  Simple LED indicators,although better than nothing (see perhaps ), may not be revealing enough for keen investigations. Full current flow monitoring (both INTO & OUT OF a solar setup), perhaps via a cheap (US$5-$10)  pocket style DMM (digital multimeter), can be near essential in many setups.

An ammeter can give valuable system insights!

Undercharge  it can reveal slight changes in panel performance due to air quality, cloud cover, aging, placement, inclination or seasons and also drop offs arising from such tedious issues as leaf/dirt/bird droppings or corroded & suspect connections.  Naturally panel damage (or even theft - it happens!) will be quickly spotted too. The relative merits of assorted batteries and panels that may become available can be  investigated as well.

Discharge  performance checks of new devices (or alerts to excessive or un-authorized loads) can be quickly noted, while longer term battery degradation can also be monitored.

Step 1: RC Electronics WU100-BK "Watt's Up" Watt Meter

OK -for serious monitoring work a Watt meter just HAS to be considered !  Perhaps the best known is RC Electronics ( => )  popular  "Watt's Up"  Watt meter, (~$US50 via Amazon/EBay etc), which is held in very high esteem. Although intended for  RC (Radio Control) enthusiasts, it's considered a valuable diagnostic & monitoring aid for all manner of  battery charge/discharge applications. Professionals should get one !

However this is an Instructable after all, so follow on for DIY approaches !

Step 2: Classic Centre Zero Ammeter

Compared with a classic "needle" ammeter, the appeal of using a "number" DMM for solar work relates to not just their cheapness & clear display, but also their ability to show both charge or discharge!  A classic centre zero moving coil "battery meter, once found on cars but now elusive"( or costly), would otherwise be needed for such an informative display. Naturally the resolution on such an analog meter is VERY coarse too!

Step 3: Pocket DMMs

At least here in New Zealand, cheap "pocket" DMMs (digital multi-meters) are of 2 types. The red UNI-T UT20B version draws ~1mA & has long been a Dick Smith Electronics offering, and although now elusive it's still stocked by Auckland based Surplustronics =>   Their leads are permanently connected making hook up blunders less likely. However it's maximum current range is only 200mA.

Australian/NZ outlet Jaycar used to stock the blue Digitech QM-1495 version, & their 2012 catalog shows they intend doing so again (this time with an orange Digitech QM-1502).   This Jaycar DMM has normal removable test leads,draws only an astoundingly low  ~¼ mA  and also offers a 10A  range.  These later 2 features make it the preferred type for  12 V solar current monitoring. 

Nov. 2012 UPDATE- Jaycar now  have the QM-1502 in stock, at an near disposable price of just US$5 !

Step 4:

All mini DMMs to hand take an A23 12 V battery of capacity ~50mAh ( alkaline). These can be hard to locate when needed, & are often quite costly- at some stockists perhaps even more than the meter! Meters seem tolerant of lower supply voltageshowever, and all worked normally on a 9V battery, with the 'low batt" symbol only appearing at about 7V.  

Here's Jaycar's QM-1502 opened- note the solid shunt that's used for 10A readings.  This remains in circuit, & thus allows solar charge/discharge currents to flow even when the meter is off. There seems good internal room for enhancements, perhaps auto power off?

Early 2013: Enhancement of this new Jaycar mini DMM has now been extensively considered, & it seems now THE best DMM "engine".   Refer a  fresh Instructable =>

Step 5:

Autopower off circuitry could be added internally, which additionally would allow pre slected settings to be retained , thus easing both user confusion & wear on the rotary switch. Here's one possible approach using a bipolar PNP Darlington Pair- many others exist (FETs & even microcontrollers  etc).

Update: Feb. 2013.
Further exploration of the new QM-1502 mini DMM  showed the most appealing  approach  used  a 2N7000 FET for auto power off.  Check the Instructable
just uploaded =>

Step 6: Pocket DMM - Pre Modification Insights

Although pocket DMMs are near laughably cheap (US$5-$10 range) they're well built & perform admirably as a spare. But even though the current drain is tiny ( ~1mA!) their small A23 battery ( which is often elusive) can annoyingly soon flatten if the meter is left on.

However- far from being a battery waster- the lack of an auto-power off can be handy for extended monitoring, & the 12V need allows direct connection to more normal 9V-12V supplies. Given their versatility & cheapness it's tempting to build one into a project (with suitable masking) as a panel meter!

Step 7: DMM Powering & Connection

If the DMM has a 10A range then simple series connection can be made via it's test leads.

However it was soon found that direct powering of the DMM was NOT possible if from the same supply  that was being measured. Attempts to isolate with blocking diodes were not successful, but a simple switched electrolytic capacitor gave pleasing results. As Q = I x t = V x C, then a 1 mA drain at 12V should hold up (until "low battery") for  a good 30 seconds with a 2200 -4700 uF electrolytic capacitor. This was nicely confirmed and could suit push switch operation for quick checks on battery charge/discharge currents.

Note: Problems have arisen with this switched capacitor approach, perhaps due to the meter's 3 Volt  reference.  (To allow negative voltages to be measured, ICL7106 based multimeters have the COM socket at 3V lower potential than the +ve terminal of the battery inside.)

Step 8: Shunt Considerations for Low Current Range DMMs

Meters without a high current range can usefully be persuaded by switching them to mV range & measuring the voltage drop across a low resistance shunt - perhaps a convenient 1 Ohm. The 4   x 1 Ohm resistors shown here are rated at 2 Watts each.

Such a shunt can most readily be made using the "1 Ohm method"  with a suitable series/parallel arrangement of  low value resistors. By Ohm's Law of course 1 Amp passing thru'  1 Ohm will have 1 Volt dropped across it.  Consider lower value shunts (of say 1/10th or even 1/100th Ohm - perhaps made from  nichrome heater wire), if higher currents are involved .

Step 9: 1 Ohm Method

Resistors under 10 Ohms are usually colour coded  with a 3rd band gold multiplier  ( gold = x 10^ -1). These are coded  Brown, Black, Gold , Gold  = 10 x 10^-1  +/- 5%  = 1 Ohm.

Four of these 2 Watt resistors,  (2 in parallel seried with another 2 in parallel to give ½  + ½  = 1 Ohm) should handle  8 Watts. Since Power = I x I x R this should be sufficient for  almost a 3 Amp current flow.

Step 10: 1 Ohm Shunt Arrays for Higher Currents & Lower Insertion Losses

All manner of series/parallel combinations can be made to suit, although you should try to stick with powers of 10 to avoid troubling yourself with tedious multipliers!  Naturally ensure the resistor combos can handle the worst case current flow.  A supply  fuse (or thermal trip) to the battery should of course be provided in case of  short circuits arising from degraded wiring insulation etc.

Step 11: DMM Powering & Shunt Circuitry

Finally here's the  "1 Ohm" shunt  circuitry.  It's again stressed that the shunt resistor(s) needs to be able to handle the worst case discharge current. This may only be a few Amps under solar charge but could soar if really heavy duty loads are connected (even short term!). A fuse or thermal trip should be fitted if such a prospect may arise. Alternatively make the shunt from nichrome resistance wire,although this will probably need calibration.

Note: Problems have arisen with this switched capacitor approach, perhaps due to the meter's 3 Volt  reference.  (To allow negative voltages to be measured, ICL7106 based multimeters have the COM socket at 3V lower potential than the +ve terminal of the battery inside.)

Step 12: Extension - Bike Computer Based A.h Metering

Extension:  The "in the energy bank" state of charge of a lead acid battery can of course be conveniently checked via it's terminal voltage. However other types, especially NiCd-NiMH,  have a near constant  terminal emf  and the stored energy level may be a mystery.

Monitoring current flow into the battery bank has been shown possible with a DIY Amp.hour meter based around a cheap bike computer. The idea behind this is that current in Amps is shown as speed, while current x time A.h displays as distance!  (Basic Physics - distance - speed x time of course).   Pedometers also have -ahem! - "mileage", but they suit just accumulated flow.

Step 13: PICAXE Persuaded Bike Computer Circuitry

A cheap PICAXE microcontroller was used to handle the detection and conversion - it's high level code ran to only ~a dozen lines.  Details of the technique were published in a Feb. 2007  "Silicon Chip"  article.  Refer the authors PICAXE resource site  for  insights and code etc.