Using LiFePO4 battery in "float" service - concerns? possible? - Page 1
Forum section: electronics / general

author: wb0gaz
author_rank: Frequent Contributor
timestamp: May 01, 2025, 01:28:07 am
content: This is either ham radio (final application) or power/energy, so starting here first.Problem to solve - 12V 10A is max available continuous current from vehicle accessory power jack (in a vehicle I wish to use for mobile HF operations - direct connection to battery isn't available due to owner considerations.)The radio has a typical 12V 20A power profile during transmit (where 20A could occur for a relatively long time - perhaps the TX duration of FT8 cycle).This leads to the problem to solve - a need for a device that could limit it's current draw (from the vehicle's accessory jack) to 10A, and provide a current supply (to the load) of 20A until it's capacity is depleted (in principle, the long-term duty cycle would need to be <50%, an assumption in this case.)A supercapacitor (often used in this sort of situation for high-power car-mounted audio amplifiers) isn't an option due to practically limited storage capacity.A secondary lead-acid (gel cell?) could work (I used this years ago when I had a Metron MA-1000 QRO 12V amplifier in a van, and it worked well, but it's not years ago anymore.)That leads me to consider LifePO4 battery (plus TBD to provide a safe electrical environment for said battery), hence why I'm making this posting asking about pitfalls, no-go criteria, or possible solution approaches.Thanks for any ideas and/or requests for clarifying details.Dave

author: brucehoult
author_rank: Super Contributor
timestamp: May 01, 2025, 03:14:34 am
content: Quote from: wb0gaz on May 01, 2025, 01:28:07 amInternet says this is 15 seconds.So you need 240W for 15 seconds, then it has a rest for 15 seconds.It's exactly 1 Wh of energy used during transmit, but actually half of that can be from the car and half from the battery.What is your budget?You can DIY pretty cheaply with just a battery and a smart battery charger. I think the main issue would be making the battery big enough that the charge and discharge rates are no more than 1C and preferably lower. So at least 120 Wh but better 240 Wh.I guess this is about the minimum you could use:$63https://www.amazon.com/ECO-WORTHY-Rechargeable-Trolling-Mobility-Scooters/dp/B09NB97XGLThe problem is how the heck do you charge that at 10 amps from another 12V system?That's getting into portable power station territory. They can accept all kinds of inputs from AC mains to solar to car accessory socket. A problem there is that they are mostly designed to output AC and the smaller ones tend to only have maximum 100W DC output.This is about the cheapest I know of with more than 240W DC output:https://www.pecron.com/products/pecron-e1500lfp-expandable-power-station-2200w-1536wh$599.It's got a 1536 Wh battery, so actually you could run 12 hours or so in the field without any input at all.It does 360W DC output (30A @12V), 2200W AC output, can charge from a car accessory socket at 100W (so not quite the 120W average you need) and can charge at up to 700W from solar panels.It weighs 18 kg.There are units for around $200 that have 250-300 Wh batteries and can output 600W of AC, but only have usually 100W USB-C (20V @ 5A) DC output.https://us.ecoflow.com/products/river-3-plus-portable-power-station?variant=41636514136137That can charge from a car 12V outlet at 220W, but I assume you can limit it if you only have 10A available.Kind of crazily, you'd have to run your transmitter from an AC adaptor with that one.

author: David Hess
author_rank: Super Contributor
timestamp: May 01, 2025, 04:23:46 am
content: The battery banks that I am aware of all limit their 12 volt output to 10 amps or so. They do however also limit their 12 volt charger input to 10 amps, allowing connection to a vehicle's 12 volt cigarette lighter socket without blowing the fuse. Combine one of these, even a small one, with a 120 VAC to 12 volt power supply and that will work. This is what I do right now if I need it.The vehicle's 12 volt supply varies between about 12.5 volts and 14.5 volts, so it is not really suitable for directly charging a 12 volt battery under all conditions without a switching regulator. A 12 volt switching regulator based charger could be designed that also limits its input current to be compatible with a vehicle's 12 volt cigarette socket, but I am not aware of anything like this commercially available.Another way it could be done is to chain an inexpensive 12 volt to 120 VAC inverter with a 10 amp battery charger. The inverter will not limit its DC input current, but if the charger is limited to 10 amps, then that is not a problem. Now you can use any battery that the charger will support.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: May 01, 2025, 02:48:18 pm
content: This is very helpful and provides good insight.Of course I left off several relevant details and assumptions, so here is my attempt to make a more useful statement of problem:1. The battery I have in mind (and have in hand) is this:https://www.amazon.com/dp/B0D7ZL6XK9"12V 20Ah Lithium LiFePO4 Deep Cycle Battery 2 Pack, 4000+ Cycles Rechargeable Battery for Electric Scooter, Ride On Toy, Lighting, Power Wheels, Fish Finder and More with Built-in 40A BMS"I went with this battery as while it's 20AH, the internal BMS has a 40A current limit, which reduces risk that the radio would drive the battery into BMS disconnect state.2. The FT8 scenario transmit duration was mostly to rule out super capacitor option; over longer term the available energy is 120 watts (from accessory jack) minus all of the overhead of energy conversion (ending up at the battery terminals.) Therefore, worst case FT8 usage would eventually lead to battery discharge (the above battery). That's just trying put some bounds on the problem.3. Going the DC-AC-Charger route suggested a small DC-AC inverter followed by a 5 amp LiFePO4 charger. That avoids exceeding the accessory jack current limit, but reduces power into the battery to at most ~70 watts (14.4V 5A) - this means I wouldn't harness all available power from the accessory jack, but the trade-off would be acceptable (reducing the draw-down duration).This takes me back to my original question (now that I've filled in blanks - sorry about that!) - is it OK practice to attach both a charger and a load to the same LiFePO4 battery (specifically the 5A charger, 0-20A load case with the 20AH battery?) Are there any other considerations around this lash-up? As the load would be a radio transceiver, should I expect some artifacts (noise?) from the charger and potentially impair charger operation from a widely fluctuating load (battery + radio)? As I wouldn't know the internals of the charger to any detail (retail product), I'm more interested in cautionary notes...Thanks again for the helpful input on this!Dave

author: David Hess
author_rank: Super Contributor
timestamp: May 01, 2025, 04:56:25 pm
content: Quote from: wb0gaz on May 01, 2025, 02:48:18 pmI have never had a problem using a charger and load connected in parallel to a battery.QuoteThe only way to know is to test it, just like when using an inverter or switching power supply with radio equipment.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: May 01, 2025, 05:12:15 pm
content: OK thanks! I've got some experimentation ahead of me (parts still arriving...)Dave

author: thm_w
author_rank: Super Contributor
timestamp: May 02, 2025, 10:04:04 pm
content: If you want to go cheap you can just use an adjustable buck/boost DC-DC off the 12V, use a fuse in series.Here is one but 7A max:https://www.aliexpress.com/item/1005008791117216.htmlSays reverse current protection so diode likely not needed.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: May 05, 2025, 12:34:06 am
content: Thanks for the suggestion - a CVCC converter (as a DC-sourced charger) sounds plausible; they often have ground continuity issues (=ground of the input/source cannot be tied to ground of the output/load), but in general it appears to be an approach to consider.Unfortunately, Aliexpress et al is temporarily (or perhaps permanently) problematic due to abrupt changes in the US import regime, but again I appreciate the suggestion and will see what my existing walk-in junkbox/parts bin might have on offer for experimentation.