Need help for a portable power pack

MSR01 · May 2016

Hello,
I am new to solar and found this forum while researching for the portable power pack I want to build for my ham radio. It should be as light as possible since I want to carry it in my backpack with the radio and also take a beating on what it means about batteries discharge and life span. I also want to use solar power to recharge and maximize the utilization time.

The LiFePO seems quite attractive for that but I am very confused by the controller/charger. My radio can put up to 100W RF output and for that it needs at least 20A @ 13.8VDC. I normally only need that much juice for 30% of the time (only while transmitting). For receiving it needs much less power. However, I also want to hook other stuff to the power pack such as my tablet and my phone to recharge them. Both use very few power since they take just 5VDC @ ~2A (typical USB port).

I found several brands of batteries and charger controllers but got stuck on this part. I already have the solar panel, which is an Enerplex Commandr 20, two of them, that can put 20W, 19VDC @~1.1A each. I am aware they will not be able to recharge the batteries while I'm using the radio but should at least extend the time I can use it if using full power. If able to use the radio's minimum power (QRP @ 5 to 10W) things will be much better. I'd like to be able to use the radio for three to four hours. With the solar panels helping I would expect to add at least one more hour to that. I believe the controller/charger must have what is called pass-through capability so the panels can provide energy to the battery while the radio is draining it?

My (initial) questions are:
1) do I have to use a controller/charger specially made for LiFePO batteries? - I read about some programmable ones. I'm not looking for anything very sophisticated.
2) what should be the charger/controller's capacity? At least 25A on the load port?
3) I read about MPPT types. Are they worth the extra price?
4) I also read about battery packs with embedded balancing circuits. Do the charger/controllers have to be compatible with those?
5) if hes, how to know that?
6) what should be the ideal battery's capacity?
7) any brands and models you recommend?

I asked quite a bit of questions. Anything I forgot to consider?

Thanks much in advance!

BB. · May 2016

You might want to look at Genasun for a charge controller:

https://genasun.com/

I would suggest a PWM controller (if you find one that meets your needs) unless you really need that extra 10-30% power or so in sub freezing weather--Or if you will, for example, have your station setup in the shade and the panels 20+ feet away in the sun (you put two or more panels in series/parallel for higher Vmp-array voltage and use the MPPT controller to take the high voltage/low current of the array and efficiently down convert to the low voltage/high current for charging the battery bank). MPPT controllers, in general, can have a computer interface (charging parameters, logs, etc. if you are interested).

If you go with PWM charge controller, you do not need a sophisticated charging controller--LiFePO4 batteries just usually need a set voltage for charging (no temperature compensation).

You can manually balance the cells--Probably good enough for your needs (short trips, re-balance at home if/when needed--Should not be a task that needs to be done very often--Save weight and complexity--Auto Balancing is more of interest for banks that are in 24 hours x 7 day service where you want to "forget" managing the bank for the next few years).

You would need a larger charge controller that have 25+ amp load ports (Low Voltage Disconnect)--I would suggest using a small voltage monitor/alarm instead to let you know when to turn off the radio/loads (system will generally be attended?). Save weight and cubic inches.

Say you want to use 60% of battery capacity (80% to 20% of capacity during typical cycling)--20 Amp load 30% of time, 1 amp 70% of time, for 4 hours, or ~4.97 amp average load, for 4 hours:

(20 amps * 0.30) + (1 amp * 0.70) = 12.7 Amps average
12.7 amps * 4 hours * 1/0.60 battery capacity = ~85 Amp*Hour battery capacity minimum.

Of course, there is running "better antenna" (directional antenna with higher gain on beam axis) and/or running at 1/2 power (~1/2 SU or 3 db drop in transmit power) to cut your battery bank/power requirements by ~1/2.

-Bill

westbranch · May 2016

There also are several threads here about LiFePO4 batteries.... very informative... ps they are a long read

Vic · May 2016

Hi MSR,

These types of questions have come up, here, before.

Even with Lithium batteries, it seems quite possible that you are expecting to use quite a lot of power from the batteries, with relatively limited recharge capability during your outings.

Is it possible, that you should consider using a different radio -- going to a QRP? radio?

Many 100 watt transceivers waste a lot of power when their power is turned down to QRP values. If you were to use a real QRP radio, it will often be much, much more efficient, particularly if it is a CW-only rig.

Just my drive-by Post ... errrr ah ... Comment. 73 GL, Vic

oil pan 4 · May 2016

I run the genasun on my 4 cell LiFePO4 pack.

I have also considered running a 15 amp Morningstar MPPT set for the following:
Gel battery setting (14.2V max)
No equalization
Temperature compensation by passed/disabled with a 10.1Kohm resistor on the remote temperature sense terminals.

You don't have to have a LiFePO4 specific charge controller but you do need one with enough features to put it in a LiFePO4 acceptable charging mode.

Need help for a portable power pack

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