by olcoolone on Thu, 06 Aug 2009 4:53 +0000
pootrol, the dude you spoke to is correct regarding the amp output and limiting it.
A deep cycle battery was never designed for automotive use, they were designed for stand by power applications in a controlled environment so don't expect a deep cycle battery to work or last as long in an automotive application.
A bit about battery's first.... deep cycle battery's are rated in amp hours (75a/h, 100a/h, ???a/h), generally this is calculated usually with a small load from a 100% state of charge (SOC) until the battery reaches 10.5 volts or near flat depending on battery and who is doing the testing.
A 100 amp hour rated battery will deliver a 5 amp load for 20 hours when the battery will be classed as flat at around 10.5 volts (100 amp hours divided by 5 amps equals 20 hours use), using the 100 amp hour battery with a much lower current draw of 1 amp you might be able to get 110 hours of use (making it a theoretical 110 amp hour battery) , if you were using the same battery and you drew 25 amps per hour you might only get 3 and a bit hours out of it (making it a theoretical 75 odd amp hour battery).
Deep cycle battery's don't like high power demands and starting battery's don't like low power demands over a long period, think of a deep cycle battery as a marathon runner who will plod along hour after hour with no real problems but get him to sprint and he runs out of puff real quickly and takes a while to recover....A starting battery is like a sprinter who can deliver very high levels of energy very quickly for a short period of time and recover quickly but has no endurance.
This is why deep cycle battery's are rated in amp hours and starting battery's are rated in cold cranking amps.
There are some battery's that are a hybrid type that can sort of do both OK but not well (think amateur runner)
Rule of thumb- for true deep cycle battery's is to charge the battery no more then 15-25% of the total amp hours of the battery, if the battery is for example a 100 amp hour rated battery you could charge it up to 25 amps per hour.
Bear in mind a battery will last longer (in terms of life expediency) with a lower charge rate...for example if you charge a battery at 25 amps you may be able to do it 1000 times, if you did it a 10 amps you might be able to recharge it 1050 times before it need replacing.
In the real world we don't have to time to recharge a battery slowly hence the rule of thumb theory.
A good quality deep cycle battery will come or have available all spec regarding discharge rates and cahrge rates.
A starting battery can and will accept a higher rate of charge more reliably then a deep cycle battery, nearly all vehicles will only charge a battery up to 75% SOC....why?- keeps the battery condition better, is a reliable charge capacity for all environments and conditions, allows for compensation with heat and fluid levels and the biggest one is why does the battery have to be recharged to 100% SOC when the vehicle will start with in 10 seconds of cranking the engine helping the vehicle to get better fuel economy, less noise, less vibration and more power (alternators can take up to 5+KW's of power from the engine when under maximum load).
About DC-DC charges... a DC-DC charger can do what an alternator can't...charge the battery to 100% SOC at a higher voltage more suited to the battery (think of an alternator as the old type of battery charger and think of a DC-DC charger as a new age 3+ step smart charger.
Battery's and battery charging is such a hard thing to explain over the net and make it understandable.
Not all battery's and battery types are the same and suitable for all applications.
Not all deep cycle battery's are rated to the same standards.
Not all battery's are built the same in construction and quality.
You pay for what you get.
Sure I forgot something.