nitpicking FarOutRide’s solar power page
In this article I will address some thoughts I have on this rather good page on FarOutRide’s site. I wouldn’t talk about it but it is getting a lot of fawning attention on reddit right now.
The whole first section is excellent all the way to battery types. At that point** I would not call flooded lead acid (FLA) “high maintenance”**. Indeed, St Sternwake has held forth often, referring to AGM as high maintenance, due to more rigorous charging requirements.
The Charging Profile section is good, but misses a fundamental point. A charging profile is a group of factory-configured charging setpoints. That’s better than no staged charging at all, but user-configurable setpoints are even better.
The section on Bulk Charging is a bit strange:
the voltage increases gradually (but generally not more than 13V).
Whut? Bulk charging voltage increases until it hits the Absorption voltage setpoint (Vabs).
And on Absorption:
Near 85% the battery become much more resistant to charging… to keep pushing energy into the battery, the charger raises the voltage. You can clearly observe that on your battery monitor (high voltage, low charging current).
No, the controller has been trying to get the battery voltage (Vbatt) to Vabs all along. When Vabs is achieved current is reduced so Vabs is held and not increased. This is the transition from “constant current” to “constant voltage”.
After a while (hours, usually) so little current is required to maintain Vabs (like C/100 or C/200) that Absorption is complete and Float can begin.
You can definitely see it on the monitor, but FOR is either misinterpreting or mis-explaining the process.
Near approximately 98%, the charger switches to float stage. During that stage the voltage is lowered and current is very low. The float stage brings the battery to a full charge and maintain it that way.
Float maintains the bank after Absorption is complete (ie, bank is fully charged).
Current can be low if there are no loads. Otherwise it will increase to meet load requirements. Sidenote: this dropping of voltage in Float is one reason MPPT makes more power than PWM outside of Bulk.
**Monocrystalline or Polycrystalline?** We read quite a bit about that and came to the conclusion that, these days, the quality of the solar panel (manufacturer) is more important than the type of the panel.
When nominal panel and bank voltages match (e.g. 12v and 12v) then mono is better suited to MPPT and poly better suited to PWM. This is a function of their respective voltages at max power output (Vmp). With PWM any panel voltage (Vpanel) in excess of battery voltage (Vbatt) is “wasted”, unaccessible by the controller. So a lower Vmp is better for any given power rating* and poly delivers that.
*assuming we can overcome temperature derating
...if using a MPPT charge controller, connecting solar panels in series is more efficient than in parallel right? We agree, except when we take partial shading into consideration…
That caveat applies to MPPT systems where nominal Vpanel == nominal Vbatt. With higher voltage panels MPPT can reduce the effects of shading.
**How many amps?** Charge controllers are rated based on the amount of amperage they can process from the solar panels. Solar Panel Max. Watts / Solar Panel Max. Voltage = 320W / 18.5V = 17.30A AMPS x Surge factor = 17.30A x 25% = 21.62A **Therefore a charge controller of at least 22A is required.**
This section is just weird.
The right way to size a PWM controller is based on the panels specs for Imax (max current) x number of paralleled panels. For MPPT, it is traditional to use:
rated watts / Vbank fudge factor, like 13v.
This would give us 24.62A. If I were in an area of high insolation (desert) I’d find a 25-30A controller. In the Pacific North_wet _I’d probably use a 20A MPPT, as they are tolerant of overpaneling. But that’s personal preference and not technical advice. They picked a Victron 100/30 (100v, 30A) which is a solid choice.
> > ###### 4.2.2- Automatic Charging Relay (ACR) > > An ACR parallels (combines) the van & the house batteries during charging (alternator or solar). ... Inadequate for lithium LiFePO4 batteries.
There are two types of ACR/VSR: single VSR and dual. The dual one acts as described above. The single (much more common) connects the two when the house battery is above a setpoint, often ~13.4v.
I don’t know why this would be inadequate for Li, unless they are talking about current limitations. Li doesn’t care about partial state of charge, and 13.8v is fully charged on LiFePO4 anyhow.
They bought a lovely Rolls AGM, one of the few makers of actual 12v deep cycle batteries. NOT cheap but a thing of manufacturing beauty.
And I agree with just about everything else on the page. :-) Good job, guys!