LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

[Dockhead]

OK, guys, sorry to break into your discussion from zero, as it were. I promise to ask my little question and then be quiet

I am waiting eagerly for this technology to get through the next stage of development -- you guys are alpha-testing it, really, and I have enough other things to do on board not to have to be reading individual cell voltages all the time I am waiting for a bullet-proof fully developed battery module with built in fully developed control system that I can just plug and play. You guys are getting such good results that I'm betting that by the time my present batteries are dead (they are one year old and I don't expect to get more than 3 or 4 more years out of them, since my boat is off shore power on a mooring) such a beast will exist

Meanwhile, I'm ready to take my first baby step into the world of LiFePo batteries -- I need a power source for an SSB radio, and with the lack of voltage sag of these batteries, they are ideal for the purpose.

Can you recommend how to go about putting together something appropriate?

The Icom M802 pulls 30 amps at 13.6 volts on full power transmit. According to what I have read, it really hates voltage sags. I am guess I need a LiFePo battery of about 15 to 20 amp/hours of capacity, fed with a 10 or 15 amp 24volt-12volt battery charger.

I'm thinking that one of those ready-made LiFePo motorcycle batteries would do, although I have not found them in the UK.

What do you guys think? Any advice? Then I'll shut up and let you get back to your regular conversation.

[diugo]

Dockhead, unfortunately, a 4-cell LiFePO4 pack will sit at 13.3V most of the time---not 13.6. So you may need some DC/DC conversion if 13.3 is unacceptable. The next issue is sag, which is caused by the battery's internal resistance (which in turn is the sum of the internal resistances of the four cells in series). The amount of sag will be this sum, multiplied by the 30A current. So to eliminate sag completely, you need a cell with virtually no internal resistance---very hard to find in a small 15-20Ah cell.

My older 260Ah bank has 8 millohms of internal resistance, so it would exhibit almost a quarter volt of sag at 30A. A newer 400Ah bank appears to have about 2 millohms, so its sag would only be 60 millivolts at 30A. A single 20Ah cell probably has several millohms all by itself.

Bottom line, a well-regulated 13.6V switching power supply may still be your best bet.

[mcarling]

Ideally, a 30 amp load would have a 60AH battery (0.5C) if maximum battery life is an issue, but 40AH would be ok. The largest LiFePO4 motorcycle batteries I've seen are 20AH, but I've seen LiFePO4 automobile starter batteries in 40AH, 60AH, and 90AH.

[ebaugh]

Unless you are a net control station, the transmit is only very intermittent duty. 30 Ah is sufficient and much better than a regular battery, even with the internal resistance mentioned. However, by the time you get both a charger and Li battery, you might as well get a good regulated power supply, unless the inverter is interfering with reception.

[mcarling]

Good point, ebaugh. It would probably be best to get a good DC-DC converter set to output 13.60V.

[deckofficer]

Dockhead,

Your idea of having independent, stand alone power for your radio is used on all the ships I sailed on. Should you incur major flooding and loss of power, you are still able to use the radios.

I never liked the voltage sag of LA, plus voltage drop on long cable runs so on my last boat I just used 2 power supplies run off the inverter. A 30 amp switching PS for the radio and a 70 amp heavy transformer based PS for the solid state amp. Now with LiFePO4, my next boat will be set up like the ships.

I believe this battery has the BMS built in because it uses the same 40 amp cells as the single cells but has been de-rated to 1C discharge, while a 40 amp cell is good to 3C.

Your radio was designed to be happy with the 12.5 volts of a resting lead acid battery, it will do better on the over 13 volts of these batteries.

Balqon - Advanced Transportation Solution

[deckofficer]

Problem with using the DC~DC converters is the RFI they give off, making it hard to hear the other station over your own generated static.

[deckofficer]

To keep your stand alone LiFePO4 radio battery charged, you won't be able to use a line from your LA housebank because its voltage will be lower than the LiFePO4 and all that will happen is your LiFePO4 will give off power to the LA.

A small LiFePO4 charger like this KP-W(120W) Charger [KP-W(120W)] - $27.00 : EV Assemble, LiFePO4, Electric Bike Conversion Kit, EV Charger, BMS, EV Components, EV Parts, All for EV! for $27 is all you need. Anytime your inverter, generator, or shore power is running, your charging the radio battery. SSB service even though pulls high power on voice peaks, average power concumption is quite low. If it was 60 years ago and we were still using AM, it would be high.

[mcarling]

Originally Posted by deckofficer

Problem with using the DC~DC converters is the RFI they give off, making it hard to hear the other station over your own generated static.

Then either put the DC-DC converter inside a copper mesh faraday cage or use a LiFePO4 battery. Using a more capacious battery will reduce the (anyway slight) voltage sag when drawing a 30 amp load.

[deckofficer]

Originally Posted by mcarling

Then either put the DC-DC converter inside a copper mesh faraday cage or use a LiFePO4 battery. Using a more capacious battery will reduce the (anyway slight) voltage sag when drawing a 30 amp load.

From my limited experience with my new LiFePO4 cells, compared to LA, there won't be much of any sag. When I first got my HAM license in the 60's, I participated in Field Day and remember the sag of LAs under transmit.

I think Dockhead's idea is a good one. There are so many sources of RFI on a boat, compressors, etc., that if working a weak signal it would be nice to shut down everything and just run the SSB on the LiFePO4.

[Dockhead]

Originally Posted by deckofficer

To keep your stand alone LiFePO4 radio battery charged, you won't be able to use a line from your LA housebank because its voltage will be lower than the LiFePO4 and all that will happen is your LiFePO4 will give off power to the LA.

A small LiFePO4 charger like this KP-W(120W) Charger [KP-W(120W)] - $27.00 : EV Assemble, LiFePO4, Electric Bike Conversion Kit, EV Charger, BMS, EV Components, EV Parts, All for EV! for $27 is all you need. Anytime your inverter, generator, or shore power is running, your charging the radio battery. SSB service even though pulls high power on voice peaks, average power concumption is quite low. If it was 60 years ago and we were still using AM, it would be high.

Thanks. My house bank is 24v, so my idea was to use a dc-dc multistage battery charger which will provide the correct charging voltages.

If the charger is providing 15 amps, then a 30 amp load will only take 15 amps out of the batt, 1C for a 15 amp/hour motorcycle batt, and that only when transmitting at max power. I can run the charger from battery power, if I'm transmitting, even if I don't have any charging source working (underway under sail, say). I wouldn't think I would need a batt any bigger than that, would I?

[deckofficer]

Even if you were Net Control, on SSB your average wattage draw is a lot less than 15 a-hr at 12 volts. So yes, a charger at 15 amps is more the adequate.

Receive takes about 16 watts.
Transmit takes around 200 watts on voice peaks, but average less than 75 watts. If you transmit the same time amount as receive, then average draw would be 45 watts. A vast improvement over the AM days.

You could do it with just a 5 amp charger.

[deckofficer]

BTW, I wouldn't use a LA motorcycle battery, as this is not a deep cycle design and won't last too long in that service. The 40 a-hr LiFePO4 I posted a link to would serve you well. If your not ready for LiFePO4, then a Odyssey AGM, which has the dual personality of being both a cranking (good for peak amps) battery and a deep cycle (good for a lot of cycles) battery will suit your needs. If the one I linked for you has a built in BMS, which I think it does, that would be perfect. I'll probably use (4) of the 100 amp LiFePO4 cells to power both the radio and the amp, but if I just wanted the radio powered, the 12 volt 40 a-hr LiFePO4 battery would be the way I would go.

[Dockhead]

Originally Posted by deckofficer

BTW, I wouldn't use a LA motorcycle battery, as this is not a deep cycle design and won't last too long in that service. The 40 a-hr LiFePO4 I posted a link to would serve you well. If your not ready for LiFePO4, then a Odyssey AGM, which has the dual personality of being both a cranking (good for peak amps) battery and a deep cycle (good for a lot of cycles) battery will suit your needs. If the one I linked for you has a built in BMS, which I think it does, that would be perfect. I'll probably use (4) of the 100 amp LiFePO4 cells to power both the radio and the amp, but if I just wanted the radio powered, the 12 volt 40 a-hr LiFePO4 battery would be the way I would go.

No, I was talking about a LiFePo motorcycle batt.

The one you pointed to has more capacity than I thought I needed, but it's cheap! I guess it would work well. Does it have built in BMS? I guess I can call them. And how do I get it to the UK? I could take it in my baggage, but I guess it's hazmat and verboten?

[hellosailor]

ebaugh-
One big problem with your circuit. If both switches are engaged...oopsie. That's an application where a double-throw-center-off switch should be used, or something similiar, so it is physically impossible to close both contacts at once.