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12V for Amateur Emergency Power

Greg Troxel, N1DAMgdt@lexort.com

Version 1.01Copyright 2016

CC-BY-NC-SA 4.0

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Meta

● This is a brand new talk– Critique welcome

– Probably it is too long

● Aimed at those new to batteries/solar– Please ask if you don't understand something or if you

think I'm dangerously wrong.

– If you have nits or details to add, please make a note and discuss at the very end or email me. I don't want to rathole since that won't serve the new people well.

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Introduction

● Powering 12V equipment in emergencies

● Focus is on Lead Acid, Solar

● Safe wiring

● Testing/charging batteries

● Examples, experience, rules of thumb

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NOT: generators

● Heavy, expensive

● Need to store, treat, rotate fuel (or depend on natural gas)

● Need to exercise

● Need to start manually, usually

● But you need it for your well pump anyway!

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Survivalism, not solar socialism

● Most focus on solar is on “saving money”– Solar is not cost effective compared to grid

– But seems to be, with Other People's Money

– How much is electricity worth when grid is down?

● Grid-tied solar– Doesn't work without grid (mostly)

– No storage – doesn't work at night

– Counterexample: mixed survival/grid-tied Extreme Ham Solar● Way more involved than grid-tied system!

● This talk is about preparing for lack of grid– When power fails, I walk to my shack and turn on a radio

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Why 12V? Loads?● Much ham gear runs on 12V

– Also phones (via lighter socket USB)

– Chargers for AA, etc.

– LED lights

● 12V systems are feasible for normal hams for always-on backup● Aimed at up to 0.5A constant, a few A RX, up to 20A TX.● My system cost $400 for 150 Ah including one battery box, 155W

panel, and 2 controllers, spent over several years.– Plus connectors, rigrunners, wiring, shelf, fuses.

– My system is largish; with scrounging and free-to-good-home you can have a smaller system for less.

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Lead Acid Batteries

● Flooded type (car)– Not safe indoors (hydrogen)

– Starting batteries not suitable for cycle use

● Valve-Regulated Lead Acid (usually deep cycle)– Absorbed Glass Matt (AGM)

– Gelled Electrolyte (GEL)

● Many sizes available (GEL typically larger)– 4Ah, 7Ah, 17Ah, 35Ah, 73Ah nominal sizes

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Other batteries

● Lithium Polymer (LiPoly)– Scary: do not charge indoors, do not use unattended (for

larger sizes)

– Bare cells need fusing, charge management to be safe – not easy

● Lithium Iron Phosphate (LiFeO4)– Expensive, becoming attractive, esp. for portable operation

– Not yet common in ham emergency power systems

– Often has integrated Battery Management System

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Charging Lead Acid

● Constant voltage: 13.8V – 2.3mV/C * (T-25C)– Simple charger works

– Multistage charges faster – goes higher briefly

● Overcharge boils electrolyte and breaks battery

● Overdischarge breaks battery

● I float at 13.5ish to be safe

● Very long time to 100% charge– But 90% charge is pretty ok

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Capacity Ratings

● Ratings in Ah: so many amps for so many hours

● Capacity depends on rate! Peukert's Law– Standard is “20 hour rate”. 7 Ah cannot supply 7A for 1h,

but in theory 350 mA for 20h. Lower draw gives more capacity. Doubling capacity results in more than doubling actual capacity for the same load.

● Marketing seems to put bigger numbers on batteries; a 7 Ah size may have an 8 Ah rating. I am skeptical that one can obtain 8 Ah from it. So I treat all batteries as having the standard capacity for the size.

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Batteries with meter: 2x73Ah

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Battery Testing

● Quality control is poor (new can be half!)● Used batteries often have issues

– If device failed, probably NG (UPS, etc.)

– If replaced on schedule due to paranoia – maybe ok

– Reduced capacity (but half can be useful)

– High impedance (ok on low loads only)

– Shorted cells (danger – will hurt others in parallel)

● If you don't test, you have NO IDEA!

– 7 Ah battery I brought to FD – did not run air pump, but ok at 350 mA

– Don't claim “emergency power” unless you have actually disconected the grid

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Testing HOWTO

● I use C/20h with standard capacity– 7Ah tested at 350 mA, 35 Ah a 1.75A

– Can compare to published capacity

– C/20h rate is the industry standard

● Charge, then discharge– Charge fully – several days on float

– Constant current discharge

– End of test at 10.5V (do not go below!)

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Testing Details

● Alternate view is to use standard loads– 200 mA, 1A, 5A, 20A

– Measures ability to do what you want

– Use 11.4V as EOT if load is < C/20h● 11.4V is resting empty

– I used to do this, but now favor C/20h● And maybe higher rates

● Test each battery once/year, label with sticker

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Testing Anecdotes

● New batteries can be bad– Low capacity but otherwise ok

– One cell with low capacity (sharp drop)

● Used batteries– High impedance (ok at low load, not at higher)

– Reduced capacity

– Almost zero capacity (ok for a few minutes though)

– Internal shorts, bad cells● These you really do not want to hook up to the rest of your batteries!

● 2x35 Ah new from UPG: over 30 Ah ==> Very Good● 2x73 Ah used (wheelchair): 30ish and 55ish Ah => Usable/Good

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0.2A, C/20h=0.35A, 1A

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2x healthy 35Ah

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2x UPS pull: 1 sick, 1 useless

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Wiring

● Every battery needs a fuse very close in +

● I use powerpoles– ARES standard, genderless

– Lots of equipment available

– Definitely use a good crimp tool

● Need big wires for long runs (e.g. 10 ga)– IR drop is huge at 12V and 10A

● Conventional wisdom says limited # in parallel– At most 3, same kind, all new

– I use many more. But I watch voltage, and test them.

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Charging

● Multiple methods:– Solar (later)

– Power supply with charger circuit

● Power supply– Some supplies fail if connected while off (7812?)

– Thus, needs diode and charge controller● e.g. PowerGate

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Solar

● Panel – must be 12V type

● Solar charge controller– Must not overcharge batteries

● Costs– batteries most

– then panel and controllers

– Mounting panels can be a big challenge

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Solar Panels

● 1 kW/m^2 available in full sun● 15-20% efficient is loose mental model

– A bit more power when cold

● Ex: 55W Voc 20V, Isc 4A– 3+ishA at 13V

– 3ish at 17V (max power point)

– Details vary with light

● Have to be permanently installed/wired– “I'll put them out when it's sunny” unlikely to work...

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Solar Controller Types

● Slow on/off – off at 14.2V, on at 13.0V– Cheap, no RFI

● Pulse width modulation – high rate on/off– Duty cycle to keep output at 13.5V

– More expensive, can have RFI

● Maximum Power Point Tracking (MPPT)– DC/DC converter; runs panel at highest output

– Most expensive, RFI potential. I recommend Genasun.

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Panel Placement

● Put them in the sun!– Easier said than done

– Think about trees and winter. Solar works great if panel has no snow and is not blocked by trees.

● Think about how to physically mount and run wires.

● Consider how many hours/day they will get full sun, especially 11/20 to 1/20 (my tough time).

● Cloudy-bright/shade is 10-15% power of full sun – With big enough panel, this is very useful.

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Example (remove snow!)

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Monitoring

● I recommend an obvious voltmeter always visible.– Lower than expected voltage can mean lack of charge, too

much load, or failing batteries.

– Should hit float in sun during most sunny days

– 12.7ish evening is good. 12.3ish is ok after several rainy days. 12.0ish means you aren't keeping up.

● Integrating ammeter, daily records– Measuring combination of production and usage, not just

production.

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Meter reading

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Cumulative Ah – linear fit

The graph shows cumulative Ah by day, minus a constant number of Ah/day.

Onset of low sun (11/20ish )is at end up upward slope. Irregular is partly due to me using panels to charge a different battery.

Steady decline is from lack of sun and lower charging.

Leveling out is mid-January when sun on panel starts to pick up.

The real issue is getting enough charge in the worst time to not discharge the batteries. Getting to float voltage on most sunny days is the real metric.

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How much do you need?

● Constant loads are bigger than you think.– 100 mA is 2.4 Ah/day

● Need to measure, evaluate, adjust.

● Think in Ah/day of usage, charge.

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Constant loads

● Real data– Kenwood TM742A, off: 30 mA

– WX radio, standby: 30 mA

– Display voltmeter: 7 mA

– Integrating ammeter: 7 mA

– Lighter socket USB: 3 mA

● “Nothing on” adds up fast, if you have a small system (single 35 Ah is small!)

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Other loads

● Calling ARES net 2m/15W for 15 minutes– About 1 Ah

● FM rig RX: 400 mA TX: 8A high power

● KX3 RX: 300 mA

● K3 RX: 1 A

● You should measure all your stuff!

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So, how much do you need?

● There is no right answer.

● With any setup, it will last sometimes, not other times.

● You don't know what's going to happen– So really you are gambling with a variable safety factor.

● If you never use grid power, you have more confidence in your system.– I unplugged my radios from the grid in February 2013.

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Rules of Thumb

● Size batteries to go 7 days– Take daily usage, *7. Then *2 to only use half the batteries. So for 2

Ah/day, a 35 Ah battery that is really 30 Ah is ok. Smaller than that is probably not ok.

● Size panel so daily charge needs are met with half the available sun hours in worst season.– With 2h of sun, need panel to produce 2A in full sun for 1h

– 20W panel iffy for 2Ah/day. My 55W averages 4.5 Ah/day but doesn't quite keep up 11/20-1/20.

● Recharge Ah is greater than usage; batteries are like that.

● You will leave power on the table all the time!– Get over it; the alternative is running out when it rains.

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Evolution of my system

● Started with 2 x 35Ah, 20W panel, sunforce on/off

● Added Genasun GV-5 MPPT controller– Got 30% more power in full sun, even bigger boost in non-full sun

● Switched to 55W panel (actually have seen 55W in full sun at good angle with MPPT controller)

● Added 73Ah GEL, x2 (Thanks Dan!!)

● Added 100W panel with sunforce on/off controller– To help with 11/20-1/20, my bad time due to trees

● Have generated just about 5000 Ah over 35 months.– Routinely charge phone/tablet, AA, HT, run mobile rig and QRP HF

● Check into ARES net “emergency power” with a clear conscience!