[Elecraft] OT: Battery and charger recommendations for 50 watt solar panel

[Greg Troxel]

Wayne Burdick <n6kr at elecraft.com> writes:

> I finally installed a single 50 W, 12 V solar panel on the roof for
> light backup purposes -- phone/pad/laptop/flashlight charging,
> natural-power use of the KX2/KX3, etc. Connecting the panel directly
> to automotive-style USB chargers works great when the sun is shining.
>
> My 15 year old thinks it's cool. He wants me to turn the mains breaker
> off for a whole day so we can get the full off-grid experience :)
>
> To facilitate that more rigorous purpose, I'd like to add a modest
> battery pack, say 10 to 20 AH, that is charged by the PV panel without
> intervention. The charger has to be compatible with simultaneous
> charge/discharge, i.e. we may need charge USB devices at the same time
> the PV panel is charging the battery. The battery voltage should be 11
> to 12 V minimum after any controller drops so it'll work with the
> radios.

You said "12V" panel, so I am assuming that the V_oc is about 20V.

You have basically a few decisions:

  MPPT controller vs PWM vs on-off

  equipment directly on the battery (well, fused) vs a controller that
  has low-voltage disconnect (LVD)

  battery size

  Lead acid vs LiFEPO4

For me, the hard part is finding a place to mount panels and mounting
them, so paying a bit more for a good controller is cost effective.  I
have a 55W panel with a Genasun GV5 and a 100W panel with a sunforce
on/off controller, and about 200Ah of lead acid batteries, and no low
voltage disconect.  For years I charged solar only, and now I supplement
with a power supply and diode (soon to be better) this time of year (sun
is often shining, but angle is low and behind those pesky trees).

Given that you mention operating and not just phone charging, and you
have only a 55W panel, I recommend an MPPT controller, and basically I
recommend that anyway.  You'll get more power out of the same panel, and
this will be even more noticeable on marginal-sun days.  The Genasun GV5
is big enough for your panel.

The sunforce controller is interesting; it's <$20 and just passes the
panel to the battery until the battery reaches 14.2V, and then opens
until the battery drops to 13V (aimed at lead acid).  This is sort of
ok, but with a panel that's large compared to the battery, the high
current will cause the 14.2 to be hit and you won't fully charge the
battery.  (I'm using it on my 100W panel because I had one handy, and
really I only need that panel when the system sags over the november
low-sun bleak weeks, and then I don't see 14.2V -- but I have 200 Ah to
absorb maybe 6A.  But I should get a GV-10.)

I don't use the LVD, because some of my loads are > 5A occasionally, and
because I have batteries in two places to avoid drop under load (but all
connected).  But the GV5 has output terminals I think with almost no
drop, that will open up at 11.4V or similar.  You might as well use it,
if using lead acid.  With LiFePO4 that has BMS that does that already, I
don't see that it matters and it is probably just complexity you don't
need.

With batteries, unless you are buying new LiFePO4, you should actually
test them.  I have found used batteries to vary from useless to almost
as good as new, and new batteries to be mostly ok but some of them have
been troubled from day one.  I use a West Mountain CBA2.

10 Ah sounds low for using a KX2/3.  Here we have multiple days of
clouds sometimes.

If you can get free lead acid batteries in good shape used (wheelchair
pulls), that's a good plan.  Otherwise you should think hard about
LiFEPO4.

I would recommend that you either get:

  a bunch of 7Ah batteries that were free to good home
  one 33 Ah or more sealed lead acid (almost all are AGM, $60ish each?)
  one or more 73 Ah sealed lead acid if free to good home (often GEL)
  one 20 Ah LiFePO4 (e.g. bioenno), or maybe 12Ah ($200, $100 very ish)

The charge controller needs to match the battery chemistry.  Genasun
configures the GV-5 for LiFePO4.

Probably it makes the most sense to go with LiFEPO4 for the scale of
system you are planning, especially when you consider the voltage on
transmit will be better matched for >5W operation on the KX2/3.  While
the battery is more money, the controller and panel are the same, and I
expect the system to work better and last longer.  Plus you can get away
with not doing tests on incoming batteries.

I also like to have a voltmeter on the battery at all times, visible (7
mA draw LCD intended for cars).  Or an ADC into Home Assistant with
alarms.

Not sure how that relates to "cobbling", but if you just wire the panel
to the GV5, and put a powerpole cable on the battery terminals of the
GV-5 and plug in to a LiFePO4, you're basically done.

73 de n1dam