Battery Questions

[KD8NGE]

As usual, one ham asks a question, several answer the question, and fellow electron slingers we've never met are benefitted from the discussion!
Gentlemen, thank you for your several replies to the OP:  you have answered questions I had but never posted!

[KD0REQ]

you have to measure under load for an accurate indication.  this is why UPS will start up and only then does the microprocessor take its samples and calculate battery condition.

for standby use, a "carbon pile tester" might be a start... but that uses a resistance that is tailored to auto engine starting current.  hooking two or three resistances in series from cheap junker units on auction sites might make you a wonderful tool.  dedicated tools no doubt exist in the hands of UPS service techs closer to the right drain, but probably cost what a contesting radio does.

[K1CJS]

I have two large (~ 100 Ahr), deep cycle batteries....

First, has anyone used this unit with parallel batteries?  It is my understanding that my operating time will more than double by using 2 batteries in parallel, but not sure whether they can be charged as a single unit (e.g. what if charging requirements were out of synch)?

If both batteries are in good shape, operating time will be doubled--not more than doubled.  You can't get more out of a battery than it has to offer.  What will be slightly more than doubled is the useable amperage that can be drawn out of the paralleled batteries.

While I am working on the calculations for operating time based on advice throughout this forum, I'd still like a way to monitor real-time battery operating life.  Would something like this battery bug be appropriate? ....

Battery monitors are good for what they do, but all you really need are the battery specs and an accurate digital voltmeter.  As soon as the battery voltage falls below either the recommended draw-down voltage or the recommended voltage below which your equipment won't operate reliably, it is time to charge the battery bank.

In normal circumstances, you can side-step the need for a voltmeter by simply leaving the voltage monitoring to a quality three stage battery charger.  The voltmeter is necessary, however, for operation where you can't or don't want to have a charger connected to the batteries.

[W8JX]

If both batteries are in good shape, operating time will be doubled--not more than doubled.  You can't get more out of a battery than it has to offer.  What will be slightly more than doubled is the useable amperage that can be drawn out of the paralleled batteries.

Not true. A lead acid or gel cell battery is not a glass of water that yields same amount no matter how fast you empty it.  The higher the discharge rate. The more than is lost to internal resistance. If you place say a 15 amp load on a single deep cycle battery and time it to discharge and then multiple that times two it will be LESS time than if you parallel two batteries because the two will only see a 7.5 amp load each and will loose less power to internal resistance and heating so you will get more run time. The higher the discharge rate the more time you will gain vs one battery at a time. A 105 amp hr battery is rated at 105 at a 20 hr discharge rate At a 10 hrs rate you will get less and at a 5 hr rate even less and so on. Paralleling batteries  spreads load so efficiency is higher vs one at a time mentality.   

[K1CJS]

That may well be true--for a constant large load.  I wouldn't call a load of a couple of amps off a hundred AH battery that.  For intermittant load useage (I doubt that the gentleman will be transmitting all the time, what I said is basically true.

[W8JX]

That may well be true--for a constant large load.  I wouldn't call a load of a couple of amps off a hundred AH battery that.  For intermittant load useage (I doubt that the gentleman will be transmitting all the time, what I said is basically true.

Not really but tell you what when you can find these batteries that have same capacity regardless of discharge rate let me know so I can buy some.

Seriously, even if peak loads are intermittent, there will still be a efficiency loss during these times due too internal resistance. If you have 2 batteries you are better off to parallel them than use them one at a time. You will get more stable voltage under load and better total run time. It is a win win.

[KK4IKO]

W8JX made a good point.  When running two batteries in parallel, a bad cell in one can affect the other.  If I were going to the expense of two deep cycle batteries, I would also invest in a good hydrometer and test my battery cells every few months, monitoring individual cell health and  electrolyte level.  You can get the old-fashioned glass tube and bulb hydrometers inexpensively, or the newer ones which pass light through a prism and a drop of electrolyte for more money.  They all work.  Don't forget temperature compensation, as specific gravity changes with temperature.

I was a Navy electrician, and worked in a shipboard battery shop.  We tested stand-by batteries quarterly, and active-use batteries weekly.  Quarterly included cleaning and re-applying Vaseline over all bare metal connections to prevent corrosion, in addition to cell testing and refilling.  Fill with distilled water when needed.

The worst thing you can do is charge them up and not use them for a long time.

73 KK4IKO

[K1CJS]

All true, but you miss the point that most of the batteries sold these days are sealed cell batteries.  Yes, you can open them up with the proper tools, but if they're properly taken care of from when they're new and aren't dropped or damaged, they do last and seldom need maintenance. 

Dead cells are usually because of mechanical damage, they don't just 'happen'--although there is the rare exception to that.  The methods and materials used these days make it less likely that a wet cell battery is going to be bad from the get go.   

[W8JX]

In about 30 yrs of paralleling batteries I only one time one effect the other. A few years ago I had a cell short out in one battery and it took other battery down in vehicle to about 10.8 volts. In all fairness battery that failed was very old and should have been replaced before it failed. Typically I replace main battery every two to three years and move main to second battery position and never wait for failure. Lost track because I do not use vehicle much these days. Normally when it sits for a month or two I isolate batteries. I had not that time and it died in while parked for a while. I have another dual battery vehicle than I cannot isolate them and it has never had a problem. But again i want to stress that I never wait for battery failure because they always fail at wrong time. I change them out as mentioned above.

[WN2C]

Since you are using the pg 40, this question would probably best be answered by West Mountain Radio. Did you call them?