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Looking for design help. Answered

I want to build a dc-dc converter with input voltage from 10 to 30 volts and output of 13.5 at 400 watts. This is so I can power a 400 watt inverter from 2 batteries on my camper... I am frustrated with the inverters shutting down at 11 volts saying dead battery, and this way I would be able to drain the 2 down to 5.5 volts each before that happened to me. Hopefully there is a simple answer for this, I am not to keen on the idea of $400.00 to purchase the only one I found on the net.


A lead acid battery IS dead at 11 volts - discharging it further will destroy the battery making it impossible to recharge.

Your commercial inverter manufacturer knows this and is protecting your battery.

Interestingly - or not perhaps, I inherited several Nife Nickel Iron with sodium hydroxide electrolyte fro my Father.

now these cells had been in Steam engines so they must be from the 1960s at the latest and i guess not charged for many years. to my surprise they took a charge! although their capacity seemed low.

Sadly for many reasons I recycled them as I had little use and no storage space.

Interstingly, or not perhaps back, but I saw an article about research significantly improving Nife batteries only last week - they are slow to charge, and don't hold much, but they are capable of very high cyclings - the new research fixes the problems.

All steam trains had a huge bank of them to provide lighting in the train. They were used I understand because they needed little maintenance.

To be honest I don't know _ I always assumed a generator on the train but this Wiki article seems to indicate each carriage had it's own dynamo



At this time, lighting was powered by batteries which were charged by a dynamo underneath each carriage when the train was in motion, and buffet cars would use bottled gas for cooking and water heating... "

I was under the impression that deep cycle batteries are designed to be drained to 0 volts with out any lasting problems.

That is the battery we always used camping when i was younger and would drain it way down, like 5 or 6 volts if i remember correctly, before the generator was drug out to charge it back up.

What about AGM or gel?

info found at http://www.windsun.com/Batteries/Battery_FAQ.htm

Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates...

Although these an be cycled down to 20% charge, the best lifespan vs cost method is to keep the average cycle at about 50% discharge.

You have your answer above. in addition a Lead acid battery has a defined life span - Normally around 7 to 10 years.

During this time if the charge is maintained and the battery well looked after it will perform as expected.

BUT If you abuse it, deep discharge, leave discharged or draw consistently high current from it you will shorten it's life.

The life span is a physical issue and not really reversible.

It's time to buy a new battery.

You mistakenly think that 80%$ discharge means that the terminal volts have changed by 80%......

From your own link (excellent by the way)

State of Charge
Here are no-load typical voltages vs state of charge

(figured at 10.5 volts = fully discharged, and 77 degrees F). Voltages are for a 12 volt battery system. For 24 volt systems multiply by 2, for 48 volt system, multiply by 4. VPC is the volts per individual cell - if you measure more than a .2 volt difference between each cell, you need to equalize, or your batteries are going bad, or they may be sulfated. These voltages are for batteries that have been at rest for 3 hours or more. Batteries that are being charged will be higher - the voltages while under charge will not tell you anything, you have to let the battery sit for a while......... It is important to realize that voltage measurements are only approximate. The best determination is to measure the specific gravity, but in many batteries this is difficult or impossible.

Note the large voltage drop in the last 10%.

State of Charge  12 Volt battery (terminal volts, open circuit)

100%                12.7 
90%                  12.5 
80%                  12.42 
70%                  12.32 
60%                  12.20
50%                  12.06
40%                  11.9 
30%                  11.7 
20%                  11.58 
10%                  11.31 
0                        10.5 

And at 0, this battery is DEAD, and may not resuscitate if left for more than a few days.

As stated the battery is basically dead at 11V.

Simple solutions for longer run times are as follows:
  • add more batteries in parallel to the system
  • build a wind power generator
  • build a water power generator
  • get some solar panels
For best results combine all 4 with a good charging monitoring system and you'll get all the power you need. But a better idea would be to not rely of the inverter so much so it doesn't drain the batteries as quickly. 

I have 115 watt solar. I have 4 deep cycles on the tounge of the trailer currently. I want to be able to power the fridge and other appliences from inverters for mutiple day trips with no problem. I also could do a generator but it has to be shut off and fueled... The batteries are 115Ah.