SLAB vs LiPo Battery

I dug out my Ultramax LiPo 7AH battery today in preparation for a trip (leaving on Tuesday) where I hope to activate a summit if I have time…

Although I left it charged (I thought) over the last year it has failed. It now only powers a FT857D on Rx for about 45 mins from a full charge before it switches off. Charge control is integrated within the battery so no hope of examining individual cells etc.
I’ve dug out an old SLAB, 7Ah, seems to charge to 12.8V OK. It IS twice as heavy but the summit is not extreme. What difference in performance am I likely to notice compared to the LiPo? probably running 25W so not QRO

At any reasonable current draw a SLAB will only achieve about 50% of its rated capacity. I would charge and run it at home prior to risking lugging one up a hill.

From my limited experience more perspiration… I think Lead Acid SLAB’s fail a bit more gracefully, but I would cycle it a couple of times if you get the chance, it might have been 7Ah, but it probably isn’t any longer. There is also an effect of surface charge, giving the impression of being charged, but the reaction is only at the surface, so it will flatten quickly on load. Some of the smarter batter charges take account of this.

Good Luck


Hi Iain,
As stated by Richard & Paul, most lead-acids such as car batteries and SLAB’s can float high after charging and be a disappointment in terms of capacity, when you try to draw current because they are rated at what’s called the ‘20-hour rate.’ (Except aircraft batteries which are rated at the 1-hour rate).

In the case of your 7Ah SLAB, a new unit is specified to supply 7Ah only when it is discharged over a period of 20 hours which is in this case a .35 Amp load. (The 20-hours is in small print or omitted altogether - a bit like dBi and dBd for antennas.) As the current is increased, the total energy which can be extracted decreases quite markedly.

If you were to select a specific load (in this case 3.9 Amps) which would discharge the lead-acid (to 10.8V) in just one hour you would have to derate it by a factor of .559 and your 7 Ah then becomes a 3.9 Ah one. (See table).

They are more forgiving of ill treatment than the pernickety lipo’s however as I can readily testify just lately after testing my full compliment of the latter with similar and in some cases much worse results than yours. I have had to buy four more 5Ah ones and one of those has swelled after only one month and its capacity reduced to 60% of its capacity on receipt. It’s had just one SOTA.

I am trying my best to treat my lipo’s better. Longer term storage should not be at full charge - some say 50%, others 70% etc. If you charge in the house you should not then store in a cold shed and vice versa. About 3.3 V/ cell is the bottom limit of discharge but some say a bit less than that but you should (or likely already have) check this out for yourself.

My IC706-2G shuts down at 9.6 V but my FT817 can still be ‘annoying’ an external 11.1V (3-cell) nom li-po at 7.2 V!!. That will take it down to 2.4 V/ cell, well below the danger level where it is likely to be destroyed. I have had to add a power-line diode chain (3 x 1N5404’s in series) to help mitigate against this possibility.

Below is a Varta battery table which shows how 20-hour rated lead-acids are derated as a function of discharge time. It’s what I used for aircraft and submersible batteries in a bygone age when I used to work. The graph below is plotted from the values given.

Good luck with the activation but be prepared to sweat that little bit more!
73, John.

Above: 20-hour rated lead-acid battery derating factors for shorter discharge durations.

You can simply change the 817 display to show the voltage and practice “coitus disconnectus” when the displayed voltage drops low enough. This does require the operator not to waffle the battery into destruction by ignoring the display or trying to be a tight-wad and squeeze just one more QSO out of the battery.

You should not the on-load displayed voltage is a bit less than the cell voltage at the terminal, you should measure both so you know what displayed voltage is the stop point.

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Thanks for the info, especially on discharge rate versus stated capacity.
I have no option now but to take it and see, but its a good walk in any event.

I need to seriously look at a cheaper alternative to the ultra-max, I had hoped with the growth of drones and RC there would be more and cheaper options. £80 is expensive for something that only lasts 2 years tops but next time I shall nurture it more. Having said that it has done enough air miles to go round the world 1.5 times so I suppose I got good use out of it…
If you hear me calling from GM/SI-149 give me a call before the battery dies!

Iain G4SGX

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Buy one of the cheap alarms used for LIPo batteries and set the threshold (per cell) somewhere above the failure point. These alarms would waken the dead; I use one in my model boats and they can be heard from inside the boat over 100m away

Note, though, that the displayed voltage can be altered to match the terrminal voltage using the “third menu”.

I am somewhat surprised by this Ian. I purchased four 4AH LiPOs in 2009 and three of them are still going strong. I had a cell go down in one, so decided to decommission it. This was probably down to it having inadvertantly been drained to around 2.5V per cell on one occasion.

LiPOs should ideally be kept at their storage voltage, e.g. 11.1V for a 3S battery, 14.8V for a 4S one, which often means having to spend time adjusting the voltage after an activation. The iMax B6AC (other chargers are available) makes doing this easy. I have always tried to adhere to this principle which works for me as outings are planned in advance and there is ample time for the batteries to be charged before an activation.

73, Gerald