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Author Topic: Battery Charging Calculations  (Read 933 times)
KD9FRQ
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Posts: 359




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« on: April 26, 2019, 05:42:24 AM »

I two sets of battery banks in my shack.

Set one is two 100AH SLAs in parallel feed by a 45A RV battery charger. I have the charger hooked to one battery and the shack power distribution panel to the other.  Once a month, I run battery only for power with an FT-450 and FTM-3200 sitting in receive mode for about 4 days.

To support a recent local event, I took one battery with me as power source. The first time I tried to Xmit using another FTM-3200, the rig shut off. Another HAM had a battery tester and it showed I only had 10v in the battery.

The second bank is new and is two deep cycle marine batteries at about 50Ah.  I am not sure what AC to DC unit I will use to feed them.

But what charger should I have on both sets? By this I mean Charge Rate?  I have 6A charger but that does not seem to de doing anything when applied to a single 100Ah battery (while not connected to the other).
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N8AUC
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« Reply #1 on: April 26, 2019, 07:23:48 AM »

You really don't supply enough info to give a definitive answer for your situation.
But it sounds like you basically cooked the battery. I've had that happen to me before.

Is your SLA battery bank two individual 100 AH batteries, or smaller batteries connected in parallel to add up to 100AH?
What kind of RV battery charger are you using? Are you certain you know how it operates? 45 amps sounds awfully beefy.
Did you purchase the batteries new or used? If used, how used were they when you got them, and do you know what they were used for by the previous owner?

Generally speaking, charging SLA batteries is at least a two stage process.
Initially, the charger should be operating in a constant current mode, at about 0.1 or 0.2C. This is called "bulk charging", and where most of the work is done.
Once the voltage across the battery reaches a certain point, the charger should enter a constant voltage mode, called "float charging", until the battery reaches 100% full.

Smart battery chargers, like the "Battery Tender", operate like this, and add a third mode, where it stops float charging a full battery, and enters a maintenance mode.
This is what allows you to keep a "Battery Tender" connected to a battery all the time without hurting the battery.
In maintenance mode, the charger stops charging the battery, and simply monitors its voltage. As the battery self discharges over time, the voltage will gradually drop.
When the battery voltage drops below "full", the charger switches back to float charge mode until the battery is full again.

Note that if your battery charger doesn't have a "maintenance mode", you shouldn't leave it connected to a fully charged battery. In that case, when the battery is full,
you should disconnect it from the charger, and just check it periodically. When the voltage starts to dip a bit, float charge it until full.

Hope this helps!

73 de N8AUC
Eric

 
« Last Edit: April 26, 2019, 07:32:22 AM by N8AUC » Logged
K1VSK
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« Reply #2 on: April 26, 2019, 07:51:57 AM »

The general rule for FLA batteries is 20% of battery capacity. Batteries can’t accept a greater charge rate effectively. Anything less just prolonged recharging unnecessarily but won’t kill the battery.
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N8AUC
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« Reply #3 on: April 26, 2019, 08:04:12 AM »

Oh, duh!
You did say about the batteries.

When you connect two in parallel, take the positive connection from one battery, and the negative connection from the other.
That way, load current and charge current will be more equal from the connected batteries.


« Last Edit: April 26, 2019, 08:27:08 AM by N8AUC » Logged
N8AUC
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« Reply #4 on: April 26, 2019, 08:20:22 AM »

The general rule for FLA batteries is 20% of battery capacity. Batteries can’t accept a greater charge rate effectively. Anything less just prolonged recharging unnecessarily but won’t kill the battery.

The 20% number is good for an absolute maximum. Don't charge, or discharge an SLA battery at more than that.
If you discharge at more than a 0.1C rate, you won't hurt the battery, but you will shorten the available length of the discharge cycle.
Also remember, you can't use more than 50% of the charge stored in the battery with an SLA. This is the "depth of discharge".
If the open circuit voltage gets down to 10.5V, you're done, and you should stop using the battery until it's recharged.

When bulk charging, you can force more than 20%, or 0.2C into the battery, and it does take it.
But it also heats up the battery internally, which will shorten its service life (# of charge/discharge cycles). Heat is always the enemy.

When float charging, you usually won't see that much current flowing into the battery.
I've never seen more than about 5% of rated current, or 0.05C into an SLA during float charging.
« Last Edit: April 26, 2019, 08:23:35 AM by N8AUC » Logged
KD9FRQ
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« Reply #5 on: April 26, 2019, 08:32:37 AM »

All batteries started out as new.

I first only had one 100Ah then added the second.

I was given the 45A RV power unit when I first got my ticket. Several local HAMS use them for their shacks with several radios attached.  I have a few 2m rigs getting ready to be added and a second HF unit coming online. Dedicated to Packet, APRS, WinLink, etc...

So, I should run battery only for a period of time, then shut down and charge the batteries? Or can I leave the rigs on while charging?
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N8AUC
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« Reply #6 on: April 26, 2019, 09:49:08 AM »

All batteries started out as new.

I first only had one 100Ah then added the second.

I was given the 45A RV power unit when I first got my ticket. Several local HAMS use them for their shacks with several radios attached.  I have a few 2m rigs getting ready to be added and a second HF unit coming online. Dedicated to Packet, APRS, WinLink, etc...

So, I should run battery only for a period of time, then shut down and charge the batteries? Or can I leave the rigs on while charging?

Unless you know how clean the output of the charger is, I wouldn't use the charger on the same battery I was operating on.
Any electrical noise from the charger will end up on your signal, and that's not good. Yours MIGHT be OK, but I wouldn't bet on that without testing the charger.
RV battery chargers are not usually known to be electrically clean.

I'd use one set of batteries while charging the other. When the set in use becomes discharged, swap the batteries and recharge.

When I operate from battery power, I have meters in the circuit continuously (both current and voltage) so that I can monitor the state of the batteries at a glance.
In fact, the meters are built into my battery box, so they're there full time, all the time.
Your heaviest current draw happens while transmitting. Some voltage drop during transmit is to be expected, due to the inherent internal resistance of the battery.
If the battery voltage while I'm receiving (minimal current draw) drops below 11.5 volts, then I stop using it and put it on the charger.
Besides, my radio starts acting really screwy when the supply voltage drops below that. So if I forget, the radio reminds me very rudely.

That's the beauty of using solar power with your batteries.
If output from the panels is available, your charge controller uses that to automatically top off the battery while it's being used.
I did that for Field Day last year, and it worked really well. When we broke camp to head home on Sunday, my batteries were almost fully recharged again.

« Last Edit: April 26, 2019, 09:57:05 AM by N8AUC » Logged
K5LXP
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« Reply #7 on: April 26, 2019, 05:20:49 PM »

 
The general rule for FLA batteries is 20% of battery capacity.

Actually, for flooded batteries it's 10%.  One exception is Trojan at 13%.  AGM's have a higher charge acceptance rate though which is one of the traits that can justify their higher cost depending on the application.  In an RV the goal is to stuff as many amps into the batteries as you can to shorten generator run time when boondocking, hence the desire to have 45A or greater converters.

Quote
you can't use more than 50% of the charge stored in the battery with an SLA.

Not only can you, if you don't you're only depriving yourself of the capacity you've paid for and are hauling around.  The capacity life difference between 50% DOD and 80% DOD is so small that there's no reason to not use it and there are other reasons to make it a point to do so.

AGM's have a very specific charge profile which will be detailed in their data sheets.  In a stationary application I wouldn't be too worried about the bulk phase, it's the absorption and float modes that will determine whether the batteries are getting a 100% charge, are being chronically undercharged or damaged by overcharge.  Most commercial chargers that have an AGM mode will have the correct charge profile and should be OK.  But in the case of the 45A RV charger, unless it too has a specific AGM mode it likely is set up for flooded batteries, and they're not directly interchangeable.

Without some testing it's hard to say, but if you took a battery fresh off the charger and a short period of time later it's down to 10V and change, it sounds like it has a bad cell.  Some charge and discharge tests would unequivocally prove it but that's what it sounds like at a distance.

The plan of attack I would use is to come up with an appropriate charger, or prove the charge profile of the existing charger(s).  Next would be to individually test each battery under controlled conditions to verify battery health.  Radios are very tepid loads so 100% of new isn't necessary.  I use 80% and up as "good" and 50-80% "usable", with anything less than 50% as time for looking to replace.

Mark K5LXP
Albuquerque, NM
 


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WA1MOW
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« Reply #8 on: May 01, 2019, 08:34:54 AM »


RV battery chargers are not usually known to be electrically clean.


I would agree with this statement. In our travel trailer, when the power converter is running, I have a lot of noise. When I put a scope on it, I could not believe the hash.

I would get a good power supply and run it off of 110 VAC.
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