My battery keeps dying if I don't have the car switched on for more than say 25mins max. Would a power cap sort this but bear in mind I don't have an amplifier.. Or maybe a second battery?

You dont have a amplifier but you have too much ice?

that sound about normal mate [25mins], a split charge system is the best option with a second batt,[bit excesive like unless you got amps etc,] a power cap will make your music brighter and base tighter as the real power is drawn from the cap[base etc]but realy no benefits unless you have amps, alot of expence and more weight for not a lot of benefits.

Too much includes dvd player, headrest screens etc. I have an amp and subs but have decided to take them out as they are too heavy/limited boot space. So I should put the amp back in with a cap and this will extend my battery life?

no not at all it will reduce your battary life, amp will use more power,hang on il break it down, the cap charges it self up from the battary, so when a deep base note playes [which needs the most power] the amp drawes the power it needs from the shortest point [the cap]which then sucks its refill power from the battary,which improves sq [a cap is not a batt]when i insall a medium system for a customer i usually upgrade the batt, something like a deep cell batt [optima is as good as you will find] normal batt on coop might be say 600 cca[cold cranking amps] an optima say 1400 cca and charges up quicker,theres no real quick and cheap way of doing things i am afraid if you want to use more power you have to upgrade the source hope this helps

If I've understood this, you're using your ICE for 25 minutes with the engine off and this is emptying the battery.

Based on that, you need more battery capacity, so either a higher capacity battery or more of them. However, car batteries aren't really designed to be discharged, so you may need something a bit more specialised.

From a rought calculation, I don't see how you're managing to drain the battery quite so fast without an amplifier and bleeding ears. Have a look at the specs of your ICE and calculate the current that it'll all draw. Then you'll also be able to work out how much battery capacity you need. If you really need to use your ICE with the engine off, then you may need a non-standard second battery with some charging electronics (probably nothing too complex, you just need to have a high, but limited, current ignition switched supply) so that it doesn't drain the main battery when the ignition is off.

i think youve killed the battery discharging it personally. don't the ice guys normally have dry cell batteries that can be run flat without losing performance?