Maybe a bigger battery if it will fit..

Put any significant load on a battery and the terminal voltage will drop. 100 is quite small for an anchor winch, I'd expect it to be drawing more than 10 amps.

Think in amp hours. If it takes 2 minutes to pull an anchor up using 480 w winch. Assume it is drawing that. P=vxa, 480 divided by 12 volts is 40 amps.

You used that current for 2 minutes.
If your motor can charge at 5 amps, that is one eighth of what you were drawing, so you need to charge for 8 times the time...16 minutes to replace that charge.

So, if you up pick and move lots, short distances, you might be slowly draining your battery. If this is only day trips, then a good charge overnight when you get home, will top the battery up. No real harm done.
You then need to use a maintenance charger if you don't use the boat at least every month..

No battery likes to sit even in a partially discharged state for long. It will die.
Anything over 50% discharge at any time will cause damage.

Its a 90hp Mercury 4 stroke. The battery is 530cca 52AH.

Maybe check the output from engine charging system?

Three points -

The concept of starting the outboard and having running while using a winch/ capstain, is not about charging.
 Alternators have next to no charge till hit around 1200 to 1800 rpms.
 The reason is that if you have a dud battery, and/ or charging circuit , at least the engine is fired up to get home.

As per others above.. batteries , any type dont like being discharged and stored partly charged, espec modern batteries... In saying that I maintained they actually have a far longer life.

About a 530 cca is ok for just your engine spec, and gps, sounder few other low current items... A larger battery with greater storage would certainly help longevity using high current winches. Something the battery ppl should advise on.

Even so getting home and throw on a smart charge (ctek) 8 stage charger...an maintain that charge between trips.

 This is all assuming that your battery connections, isolating switch connections etc, the battery top is kept clean and that the charging system is actually working.
These will not only cause extra current draw to items but also effect the rate the alternator re charges.

In general a fully charged battery will give you 1 1/2 good days trips out fishing before you notice that when you trim the engine it sounds different and weak.

Check  and clean, re grease connections, then check its charging...

letsgetem wrote: According to the manual (for the Mercury 90hp), the alternator charges at 50amps. When its charging, the voltage rises from 12v to 14v.  And I can confirm the motor alternator does not charge until over 1000rpm, ie not at idle.

Mate the reason for starting the motor first is if the battery get to flat to start the engine while your running the winch.

approximate

Good stuff, thanks.

BH covers that above...
just put it on a maintenance charger once a week or simply leave on the smart charger.
 And make sure your chargers are the correct charger for the type of the battery, otherwise you will over charging a little or undercharging and either of these also drops the battery life dramatically.

 Also that 50 amp rating is not 50 amp at 1000 rpms. The 1000 rpms is where it starts to charge.
 
There is also another long term issue with a battery too small for the engine , and or too small for the engine and other large loads at the same time... If your voltage drops too low, you CDI / ignitions sealed regulators  have to draw more current to maintain their required power draw.
 More current means they then heat up, and they do not like running too warm or hot.
 And they are expensive to replace...

Never run the engine disconnected from the battery either.. that kills the sealed regulator/ tach unit, and they are not cheap either.

From memory my manual says run a min 630cca battery
I have a 760 or 780..because running high pressure wash down pump or the capstan from 50/ 60m.
 The Local auto electrician only has 2 rather 'oversized ' marine batteries in stock. Reason being he doesnt get ppl coming back because of early battery failure.