Difference in Ah of same voltage batteries and costs

Discussion in 'Off Grid Living' started by rsbhunter, Aug 5, 2012.


  1. rsbhunter

    rsbhunter Monkey+

    Ok, need the help of the guys here that live and breath this stuff (TAndy and BTPost) and any others that know the answer....Was looking at Rolls batteries, and noticed that for $350.00 you can get a 400 Ah 6 volt battery (i will need probably 8- 2x4) for 24 volt system. OR if you want to spend more $$$(1200.00) you can get a 6 volt battery that has 820 As . Is there an advantage of using 4- 820 Ah batts over 8 400 Ah batts,other than the obvious cable and terminals, etc, IF the cost was equal....I know there not the same amount of $$$$, but was curious if it was a benefit one way or the other....Also, would a person be better off to use 2-12V batts, 4-6V batts , 3-8V or 12-2V batts for a 24 volt system, given comparable Ah's capabilities....This might be a really stupid question, and i honestly apologize if it is, but i am curious...Thanks (or sorry) rsbhunter
     
  2. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Not so stupid...... The difference between battery cells that are 400AH, and ones that are 800AH, is power available for use, and still not exceed 20% Discharge. It is better to have a single string of cells in Series, than two strings, wired in parallel. So, all other things being equal, a larger AH cell, in a single string is better that smaller AH cells, connected in parallel. If you need more power, it is better to increase the Voltage of the System, by adding cells in series, than to connect another set of cells in parallel. This does require planning, and sound system design from the start. You can always reorder your cells in a Battery Bank, but Inverter input voltages are more than likely FIXED, so changing Bank Voltages, means buying a different Inverter, which is a major expense. With modern MPPT Charge Controllers, Alternative input power can be converterd to ANY Battery Bank Voltage, and that can be changed, by reprogramming the Charge Controller, as needed. Another NOTE, here... On occasion I have seen some folks tap a 24 Vdc Battery Bank, in the middle of the string, for 12 Vdc. This is a BIG No No, unless ABSOLUTLY Nessisary, because it unbalances the Charge/Discharge cycles of the individual cells of the Bank. Much better to buy a Switching Dc to Dc Power Converter, if you need an alternate Dc Voltage. ......
     
  3. rsbhunter

    rsbhunter Monkey+

    As far as tapping in for 12v, that , from what iv'e learned is a BIG no-no.And i do have the Outback VFX 3524 FlexPower 1 system. So, if i understand what your saying, i'm better to go with 2 strings (of 2) of high Ah 12v batteries (if needed for alot of Ah), than, 3 strings of 6v lower Ah batteries, or even the same Ah, because it is more effiecient in the charging of the batteries...yes? I understand that there is a viewpoint that i would be better off with 3 strings, that if, one batt fails, you're only removing a third of capacity (1 of the 3 strings), instead of 1/2....but if the batteries are maintained faithfully, and not brought under 80% of full charge AT MOST , then the reliability of the system makes up for the loss factor of a batt going down in a large quanity batt setup....? Is there any concensus on the 2-12v/4-6v or 3-8v, etc set up in a 24v system///As always, i get the most informative answers here....Thanks, rsbhunter
     
  4. HK_User

    HK_User A Productive Monkey is a Happy Monkey Site Supporter

    Always plan on how you are going to handle the weight of the batteries. This means transport to the site and movement into and out of the rack.
     
  5. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Case Study:
    I have a main Cabin INVERTER (Trace SW4024) that runs 24VDC on a 1200AH 24 Vdc Battery Bank. The bank is made up of L16HD Batteries, which are 6 Vdc 400AH. So I have three Strings, of four Batteries in Series, that are paralleled. This design was initially set up for a much smaller Load, and my original Inverter was a Trace 2624SB, running on a single string of L16's. As the load grew, over then years, I went to two Strings of L16's in parallel with gave me 800AH. Then I made my FIRST Design mistake. When the SW Series Inverters first came out, I bought one with a 24Vdc Batter Buss Voltage. Had I been a little bit more forward thinking, I would have, and should have purchased the 48Vdc version (SW4048) instead., and then just ReOrdered, the Battery Bank to a single string of 400AH at 48Vdc in 8 Batteries. Same Power but a better design. Then to double power capabilities, all I would have needed to do is add a second string in parallel and I would have had 800AH at 48 Vdc in 16 Batteries. I now have a SW4048 Inverter but have yet to buy the fourth Set of L16HD's, to make a double String Bank, of 8 ea, L16HD's for 800Ah at 48 Vdc. What I have works, and therefore I guess I am just to lazy, to do the switch, now. My Trace 2624SB runs off my Absolyte II 700AH Battery Bank, and it runs my Winter Shop, where all the Servers, IP SAT Modems, and Networking Hardware live.
     
  6. rsbhunter

    rsbhunter Monkey+

    I'm thinking i did just that...i maybe should have gone the 3548 Outback setup....but, i will live withwhat i have(and i can't return it, even brand new)...so, i gather i should go with the highest Ah i can afford, that will give me the best price per Ah batteries, thereby using the K.I.S.S. theory......Thanks, i will look at more batt companies, though the Rolls -Surrett's seem to be everyone's first choice, price aside...rsbhunter
     
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