“Watching how many amps a charging battery is accepting at absorption voltage, is very indicative of state of charge, the less the amps the more charged.” – sternwake1)
If you want to see lots more amps coming out of the [charge controller], put on a [significant] load… in the middle of a sunny day. – John61CT2)
It is common for those with new solar configurations to worry about whether or not they are working. The worry is understandable because:
The last point may be counterintuitive. A fully charged system is observed to deliver very little power because the bank is already charged; the system is loafing, waiting for a load. It only cranks up power when you add a big load or start charging a depleted battery. With lead chemistry batteries this also applies in Absorption stage, when the battery reduces the amount of current it will accept.
A poorly running system is observed to deliver very little power because it isn't set up right (or is shaded, etc) and can't do any better. Battery voltage is too low3) and doesn't meet needs.
Also counterintuitively, systems with lots of panel start generating usable power so early after sunrise that they can finish bulk charging well before solar noon. This means the user might never see the system at full4) power during normal circumstances.5) This effect is particularly strong when the system is overpaneled.
This page is to help solar beginners tell what their system is doing without special equipment. A battery monitor can be extremely useful but we can tell a great deal without it. The info here is oriented to typical 12v systems with lead-acid battery banks. Folks running other battery chemistries or nominal voltages are assumed to already know what they're doing.
Icons and blinking lights are often misleading or oversimplied. Tracer/Renogy controllers are infamous for confusing battery graphics. Others like the Victron 75/15 accurately show which charging stage is running.
Measurements, on the other hand, are useful when we learn how to read them.
To tell at a glance what your system is doing you need to find a couple pieces of info about your system. You will only have to look them up once; it might be worthwhile to write them down on a sticky note near your solar gear.
The specs we are interested in are:
On the curve pictured at the right Vmp is at the peak of the red voltage line, and Voc is on the far right where output crashes to 0W.
If you have an MPPT controller also look up these pieces of info:
These checks are admittedly crude but will help see if your system is getting it done. No expensive or specialized equipment is required.7) MPPT controllers in particular will reveal a great deal of information by how they interact with the panels. For the purposes of this article, consider single-stage/shunt controllers to work the same way as PWM except they only have one voltage setpoint.8)
For the examples below we will assume the system is set up this way:
Vpanel - measured voltage of panel output
Vbatt - measured voltage of battery bank
During bulk charging with MPPT
During bulk charging with PWM
During Absorption charging batteries will need less and less current.
with MPPT controllers
with PWM controllers
When transitioning from Absorption to Float the voltage needs to drop about a volt. The system will “free-fall” (make little or no power) to allow the voltage to fall.16) The transition may take seconds or minutes, depending battery chemistry and how/if the system is loaded.
Configurations with no float (“charge and stop”, found on some Lithium profiles) will
The fall to reBulk looks like the transition to Vfloat described above. In both cases the controller makes no (or practically no) power until the lower setpoint is reached.
A constant cycling 14.4v→13.2v→14.4v might seem extreme but in practice there is little actual cycling occurring. A fully-charged 4S LFP with no loads will rest somewhere around 13.6v. So for actual SoC changes we are talking about the difference between 13.6v and 13.2v. With significant loads applied 13.2v observed could mean SoC as high as 90% and with trivial loads as low as 70%.18) In normal use the real cycling might be 100%→85%-100% and the solar is helping carry the loads during charging periods.
By default Renogy Li profiles work this way, and cause much concern for Renogy users who have not read their manuals and/or who are not familiar with how solar charge controllers / chargers work. [The required information is present in the manuals but Renogy really should spell it out for first-timers. – secessus]
During Float with MPPT
During Float with PWM
Adding a load can help reveal how much untapped power your system can access, and also prove that your system is working.
When adding loads to MPPT during absorption (or float)
When adding loads to PWM during absorption (or float)
These are general targets to ensure the system is working at the bare minimum to meet your needs. They give a big picture but no information about what is happening at a particular moment.
If not, address this issue soon because discharging too deeply will damage the batteries. Consider these changes:
Are your batteries starting Absorption by noon-ish and completing Absorption by late afternoon?31) If so, continue to the next step.
Note: You can tell your batteries are in Absorption because the bank will be at Vabs, the controller's absorption setpoint.
If not, delay running heavier loads until Absorption is underway.
If not, either we are not harvesting enough power or we are using too much. Stop running heavier loads when the controller can no longer hold Vfloat.
If you are seeing sudden voltage spikes and have lithium batteries, you may be seeing the BMS disconnect the charging circuit. When this happens the controller suddenly finds itself making too much power (the charging demand suddenly disappeared); voltage on the rest of the circuit will spike while the controller reacts to the new demand level.
The fix, described in the link above, is to charge at a lower voltage that does not antagonize the cells and trip the BMS.
An MPPT controller must sweep the Power Points along the array voltage curve in order to Track the Maximum (and non-maxiumum) power points. Basically the controller is asking “How much power does it make here on the curve? And here? And over here?” It tracks (remembers) these power points and can return to them as needed. But solar conditions are always changing so the power point data can get stale quickly and the controller has look again from time to time.
Depending on the algorithm the sweep may a small one near presentVpanel, it may be a full sweep from 0v-Voc, or anything in between. It may happen often or seldom. Victron famously makes a sweep every 10 minutes, resulting in visible dips in the app's graphs (see orange line on image at right).
The sweep will interfere with power production so the mfg attempts to find a balance between