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#1 proud clamper

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Posted 23 September 2020 - 01:30 AM

I am looking at a power source for my DC powered gear in the 100Ah range? A deep cycle lead acid,  AGM, GEL, or a Li battery? 

I have read that the Li batteries don't do so well in colder temperatures but have more recycles and are on average 40% lighter than the others. They also have a lower depth discharge (down to 15%) as compared to 50% for lead acid and are more efficient so therefore a faster recharge.  And then there is the price difference, but if they last longer then they would be the best bang for the buck right?

If anyone is familiar with these batteries and could give me recommendations on which they prefer I would be much obliged. 

 

 

 

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#2 luxo II

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Posted 23 September 2020 - 01:48 AM

I've opted for a deep-cycleLiFePo battery which I'm building into a spare Pelican case (a clone not a real one) with the associated wiring, fuse, switches and plugs & sockets. The idea being the plastic case helps stop the battery getting cold on winter nights - and in addition I've left a space all around it with the aim of inserting 1 or 2 chemical hand warmers under the battery to keep it cosy; those things run for 6+ hours once opened.

 

The other reason to opt for a deep-cycle LiFePo is that they maintain a very flat voltage until very nearly exhausted, when the voltage falls off a cliff. The other types OTOH have a discharge characteristic where the voltage drops off progressively with the discharge. This might not matter for some equipment, such as fans or heaters, but its a royal pain with my AZEQ6 mount which shuts itself down on the dot at 11.4V.

 

The other advantages of a LiFePo is it's about half the size and weight of a comparable SLA and their life is rated at far more cycles, so it should last longer.

And if you do go LiFEPo its essential to buy the matching charger,


Edited by luxo II, 23 September 2020 - 01:52 AM.

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#3 philinbris

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Posted 23 September 2020 - 02:19 AM

Li batteries also have an average 12.8 VDC which is much better for the AZEQ6.

I have two 18AH ones in parallel and my mount lasts all night long (no heaters or other equipment like a Pi or laptop though).

Cheers



#4 luxo II

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Posted 23 September 2020 - 02:37 AM

Phil I was using a cheap, small 12Ah SLA battery to power the AZEQ6 alone, and that worked OK for a few years (just replaced it) though it is significantly discharged by 11-12pm and has to be recharged the next morning.

But it can't do 2 short sessions on consecutive nights (I've tried).

 

I now have an ASi533 and that's a whole other matter



#5 Phil Sherman

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Posted 23 September 2020 - 06:09 AM

Lithium batteries have the highest power density but suffer from changing voltage during discharge. They are also generally rated at around 500 cycles life when used in full discharge cycles. LiFePO4 batteries have an almost flat discharge curve, are rated at 3000 or more cycles and have a slightly lower power density than lithiums. Lead acid batteries are relatively cheap, heavy, and have some voltage change during discharge. They also suffer from voltage drop as the load is increased, a result of the battery's internal resistance.

 

All batteries are effected by low temperatures. Lithium types all work to some temperature slightly below freezing. Lead acid batteries suffer from significantly decreased capacity as temperatures approach freezing.

 

I just replaced the 17Ah AGM lead acid battery. In my portable power supply with a 40Ah lithuim. The lithium's voltage during a full discharge cycle will drop to 10V or less. This will not bother my Atlas (Orion's Skywatcher EQ6 clone) mount because my power box has a buck/boost regulator set to 13V between the battery and the mount. I expect that the lithium battery will have four times the usable capacity of the older AGM one.

 

One issue that needs to be addressed when buying a lithium based battery is the maximum continuous discharge rate. Lithium chemistries can be adjusted to provide low, medium, or high discharge rates. This rate needs to be factored with the cell configuration of the battery to determine the continuous discharge rate if the manufacturer has not specified it in their specifications for the battery. The BMS included inside a lithium type battery should limit the discharge rate to a value that is safe for the cell configuration.




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